HD Series Power Modules Application Guide (Revision 2)
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INTRODUCTION  

This document is intended as an application guide for STARVOX HD series of DC to DC Converter modules. These notes are also applicable in most cases to other types of encapsulated DC to DC Converter modules since the input/output and system interfacing requirements are similar if not identical. For custom or system specific applications not covered in this guide, please contact an applications engineer at STARVOX for assistance.

It should be noted that HD series or DC to DC modules as with most high "density modular converters, are NOT complete, self contained power supplies for many applications. The DC to DC module can effectively be thought of as a high power, high efficiency, "four terminal" regulator analogous to the low power, three terminal linear regulators that are widely used. As such, many applications may require additional input "front ends" and / or output filtering to meet EMI, ripple or noise requirements. Certain applications may also require circuitry to perform control, monitor or other "bell and whistle" functions such as remote inhibit, DC Power OK, power fail, etc. The following technical notes should provide the user with a practical working guide for the more typical applications of the HD series of DC to DC converter modules.

INPUT CONSIDERATIONS  

AC Input System : For Systems requiring off-line AC inputs, an AC to DC "front end" will be necessary to provide either 155 or 320V DC to the converter modules. For use with 115V AC inputs only, a simple bridge rectifier with filter capacitor, as shown in Figure 1, will produce 160 VDC output for the HD 155 series module. Applications requiring both 115 and 208/240 VAC inputs should used the dual input "strappable" off line front end shown in Figure 2. This latter configuration will produce a nominal 320 VDC bus for used with the HD 300 series module. For power levels over 300 watts in European applications it may be necessary to utilize a front end with power factor correction circuitry. Such front ends are commercially available from several vendors.

As shown in both Figure 1 and 2, an EMI filter should be used to attenuate conducted line interference for compliance to FCC conducted emissions requirements. These filters can be custom made or purchased from commercial manufacturers such as corcom or Potter Brumfield. Individual fuses should also be provided on the positive DC input for each module used in the system, in additions to fusing or a circuit breaker on the AC input side of the system front end. The maximum AC RMS input current for such a system can be calculated from the following formulas:  

Irms - Pwr out max/ (Effic. * PF * Vin min)

Where the efficiency can be taken from the specific module’s data sheet, the power factor (PF) can be approximated to 0.65; and Cin min is the minimum (low line) AC operating voltage. The final value of the AC input current should be multiplied by 1.2 when sizing fuses or breakers in offer to comply with U.L. requirements and for derating to prevent potential fuse fatigue at elevated ambient temperatures. The RMS input current value calculated above will also help in selecting the appropriate EMI filter and the current rating of the input rectifier. For 115/240 strappable systems the current rating should be figured for 115 VAC input while maximum voltage ratings should be figured for 240 VAC. For such systems a 600 volt diode bridge is recommended for maximum reliability.

A reasonable input capacitance (Cin) value for an AC front end can be approximated from the following relation :

Cin (uf) = (Irms * 8000) / Vdc nominal  

Where Vdc nominal is the nominal DC bus voltage produced by the desired front end configuration. In the dual input configuration of Figure 2, the calculated value of Cin represents the series value of the capacitors A and B, so the value must be doubled to arrive at the proper value for each capacitor.

Example : Dual (strappable) input frond end used in 208 VAC

Mode for a single HD 300-5 module :

Irms = 200w / (0.8 *  0.65 * 180 VAC) = 2.0 amps max.

Cin = (2.1 * 80000) / 300vdc = 560uf (series value), so select Cin A and B = 1000uf at 200v each.

Determining Cin by the above method will also allow for a minimum hold up time (assuming the line fails from nominal value) of 20 milliseconds or better. Less front end input capacitance can be used, however, values less than about 2/3 the recommended value will seriously degrade system low line and / or brownout performance.

Suitable filter capacitors for AC input sections include the Nichicon LK series, the Aerovos M LPR series, Sprague 80 and 81D series and the United Chemicon KME series.

Initial turn on inrush current limiting for the input section is provided by NTC thermistor TH1 (in figure 1
and 2). A suitable device for this function can be selected from the "Surge-Gard" line of thermistors from Amatek (Rodan Division).  

DC Input System : For systems with remotely located DC power source the input bus should be well filtered and represent a fairly low source impedance to the DC to DC module. Input fusing (or breakers) for protection and some minimum input capacitance should be provided. The input fuse value can be found by calculating the DC input current :

Iin = Pwr out / (Effic. * Vin min)  

Where Vin min is the minimum specified input operating voltage to the module. The fuse should be sized approximately 30% to 50% higher than the input current to prevent possible failure from thermal fatigue or input transients.

The input capacitance is necessary to compensate for the increased input DC bus source impedance appears relatively high, the specified low input dropout voltage for the module could be compromised.  

The input capacitor should have a low ESR (equivalent series resistance), high ripple current construction such as the Aerovox M LPR types or the Nichicon LK series, etc. (mentioned in the AC input section previously). The capacitor should be located as close to the module’s input terminals as possible and each module in the system should have its own input capacitor. For some systems, especially those with 12 and 24 volt inputs, it may be necessary to employ several input capacitors to keep the input ESR ( and effective input impedance) down to 30 milliohms or less if the minimum input dropout voltage of the module is to be realized. Depending on the type and the construction of the capacitor, the resulting capacitance may be in the order of several thousand microfarad. The ESR ratings of most low impedance capacitors are usually given in the manufacturer’s catalog data sheets along with their voltage and capacitance ratings.

Other optional input considerations are shown in Figure 3 and could include EMI filtering and / or reverse polarity protection. Reverse input polarity is accomplished by diode D1 which conducts and opens the input fuse (or breaker) if the DC bus polarity is incorrect. The input EMI filter is usually of the common mode type utilizing a high permeability ferrite input balun inductor in conjunction with common mode (line to chassis and) capacitors. For situations that could encounter high voltage, short duration (microsecond) input transients, this input configuration is a must, and may additionally require across the line MOV or Transorb type suppression devices.

In other cases where the input bus is "clean" or semi-regulated, only the input like to chassis ground capacitors are necessary. Both the EMI filter and reverse polarity diode could serve multiple DC to DC modules if rated accordingly.

Output Capacitance : The maximum output ripple for the STARVOX HD series of modules is specified at 3% maximum. This value is generally typical of the lower voltage, high output current versions and will be less as the rated output voltage increases and the current decreases. For systems requiring lower ripple one or more low impedance electrolytic output capacitors (such as the Nichicon UPL series or United Chemicon SXF series) can be placed across the DC output terminals. In addition to lower ripple the extra output capacity will improve the transient responses by reducing the amplitude of the over/undershoot envelope. The high frequency noise component on the output can also be attenuated by the addition of low ESR dipped or molded tantalum or ceramic capacitors of appropriate voltage rating. Figure 4 shows a typical output filtering scheme for a single output module.

The high frequency output noise (as opposed to the switching frequency ripple) is composed of two components, differential mode and common mode. The differential mode component is the noise on the (+)output terminal with respect to the (-) terminal while the common mode component in on both output terminals with respect to the input bus. Proper filtering of both components is possible only if neither output terminal is "hard" connected to the system chassis ground as shown in Figure 4. In this balanced configuration the common mode noise component is decoupled to the system chassis ground via C3 and C4. If one output terminal (usually the negative) is connected to the system chassis ground, the common mode noise will now become differential mode and additional high frequency decoupling capacitors may be necessary between the (+) and (-) terminals.

Measuring Ripple and Noise : The output ripple and noise from encapsulated types of DC to DC converters (and any switching power supply for that matter) CANNOT be accurately measured with a conventional scope probe. This type of probe is high impedance, unbalance, and the ground clip wire loop will pick up radiated RF noise and add it to the output ripple envelope. A more accurate method for measuring the module’s output ripple is with differential FET probe, or by constructing the simple but effective coaxial probe shown in Figure 5, This "ripple probe" is comprised of a 1 meter length of RG 58 coaxial cable (52 ohm) terminated on each end with a 51 ohm resistor. The terminating resistor at the probe sensing end is in series with center conductor since the power module output will typically have an output shunt impedance of no more than a few ohms. The output (scope) end is AC terminated through a BNC "T" connector with a 0.1 uf disc capacitor in series with the 51 ohm resistor. Assuming the sense leads at the probe end are kept as short as possible to minimize radiated pick up, this arrangement will make a fairly accurate probe for measuring differential mode ripple and noise. In a real world situation, the inductance of the output bus plus the decoupling capacitors usually present across each c’s power pins will virtually eliminate any noise spikes that may be present at the module’s output.  

Minimum Load : The STARVOX series of modules will require a minimum load of approximately 2% of the rated maximum output current if operation down to zero load in the system is required. Without this minimum loading the output will "high up" between the rated output voltage and zero volts.  

Triple (or dual) output modules will require a minimum load on the main channel of about 15% to maintain specified regulation at maximum loading on the auxiliary channels.

In some situations the output load may appear as a "negative resistance" at system turn on (high capacitance and/or motor type loads). This can cause the module to go into an overload hick up mode which may prevent proper system start up. If this occurs, the addition of a 200uf to 470uf, 25 volt electrolytic capacitor across the PARALLES and (-) Vin terminals (+) to the PAR pin should eliminate this problem. It will slightly increase the turn on delay of the module, however.  

Triple Output Configuration : The triple output modules can be configured in several ways. Since all of the outputs are floating with respects to another, any polarity or communing scheme is possible. Channels 2 and 3 can be used in the standard (+) and (-) arrangement, or they can be wired in parallel to produce a dual output supply with double the continuous current rating on the auxiliary channel. When used in the triple output configuration, channel 2 is capable of providing a surge current (up to 5 seconds) of double its steady state rating as long as the total power rating of the module is not exceeded.

The auxiliary channels can also be connected in series to produce a dual output module with second channel having an output voltage equal to the sum of the voltages of each separate auxiliary channel.

Systems requiring highly regulated auxiliary channels can apply three terminal linear regulators to channel 2 and/or 3 by specifying an appropriate "header" output voltage of 3 to 4 volts above the output rating of the three terminal regulators.  

Remote Sensing : The HD series of modules provide for remote sensing so that precise regulation can be achieved at the load point if necessary. This feature is particularly useful in applications where the load current varies significantly and/or the power module is located a considerable distance from the load.

Remote sense implementation is achieved by connecting the +RS and –RS pins to the + OUT and the – OUT respectively at the load point where tightest voltage regulation is desired. This is illustrated in Figure 6. If cabling is used for the remote sense connections it is recommended that a twisted pair be used to minimize noise pick up.

The remote sense feature will compensate for up to approximately a 0.25 volt drop per load bus, or 0.5 volts total. Voltage drops that exceed this value could cause the over voltage circuit in the module to trip. It should also be noted that a system will always be inherently more stable if remote sensing is not used. The remote sense lines can also be a major source of radiated noise in a system if the layout is complex. In most cases if adequate output busing is used, and the load is relatively constant the remote sense pins can be connected directly to the module’s output terminals and the system regulation will still be adequate.

Remote sensing on the triple output modules (main channel) will cause an increase in the auxiliary output voltages by the same percentage that the main channel’s output voltage (measured directly on the output pins) is increased to overcome the load bus voltage drops.

Output Voltage Trimming : The power module’s output voltage can be adjusted or trimmed over a minimum range of + or – 5% by connecting a potentiometer to the RS and TRIM pins as shown in Figure 6. Recommended potentiometer values are shown below.

The main output on the HD series triple output modules will adjust similarly and the auxiliary outputs will track the adjustment by the same percentage.

Output Inhibit (on/off) : The output of the HD modules can disabled by pulling the ON/OFF (or INHIB) pin to less than approximately 1 volt with respect to the negative input terminal (-VI). In the triple output version all three outputs will be inhibited. The switching device used for pull down on this terminal should be able to sink 2mA (per module) and saturate to less than 0.75 volts.

Figure 7A shows a recommended way of implementing the inhibit function. An optocoupler is used because, in most cases, the controlling signal will be referenced to the system’s negative bus (or –Rs). If the controlling signal is reference to –VI, then the optocoupler can be replaced by a signal level NPS bipolar transistor (such as a 2N2222).

In systems where two or more power modules are used, the inhibit feature can be used to shut down all power modules at once by "Oring" the inhibit lines together prior to the inhibit transistor or optocoupler. Low forward voltage drop diodes (Schottky) such as 1N5818 or 1N5819 types should be used for Oring the inhibit lines as shown in Figure 7B.

WARNING : under no circumstances allow the PAR or ON/OFF pins to come into contact with the +V1 pin or the module will be internally damaged. The PAR pin should also never make contact with –V1.

Maximum Baseplate Temperature : The maximum baseplate temperature of the HD series of modules must be limited to 85° C for maximum rated output power. Baseplate temperatures exceeding this will activate the over temperature circuit and the module will shut down until the temperature drops below approximately 83° C.  

Cooling : Cooling of the module may be accomplished with convection, conduction, forced air or any combination of the three. In free air the baseplate to air thermal resistance is approximately 5° C / watt. 500 linear feet per minute (1fm) of forced air over the baseplate will reduce this to approximately 2° C/watt, while 1000 1fm will drop the resistance to 1.5° C/watt. Figure 8 shows a graph of the module’s thermal resistance with respect to linear air velocity for three heat sinking configurations. One is using only the module’s exposed baseplate and other two are for Aavid Engineering’s models 4100 and 4101 custom extrusions mounted on the module.

The baseplate temperature rise (Tr) can be approximated from the following formula :

Tr = [ (Pwr out/Effic) – Pwr out ] * Thermal resistance

Where the efficiency is taken from the specific module’s data sheet (specified for 75% of max. Rated load) and the thermal resistance is deduced from Figure 8.

Example : HD300-5 with 25amp load, baseplate heatsinking only and 1000 1fm of air;

Pwr out = 5v X 55A = 125W
Effic = 80% (approximately by data sheet)
Thermal resistance (Fig. 8) = 1.5° C/W
Therefore : Tr = [( 125 / 0.8 ) – 125 ] * 1.5 = 47° C  

Assuming a 25° C ambient, the baseplate temperature will be 47 + 25 = 72° C, which is within module’s maximum thermal rating.

Thermal compound (white goo) should be used between the module’s baseplate and what ever surface or heatsink it is mounted to. This will establish an interface thermal resistance of approximately 0.2° C/watt, which will maximize cooling efficiency.

General considerations : The HD series of power modules utilize current mode control to regulate their outputs. As a consequence the output appears as a voltage controlled current source and, when operated in parallel, the modules will have inherently better load sharing characteristics than conventional voltage mode controlled power supplies. This feature, however, will not assure current sharing or even stability if proper care is not taken in the power system layout. Output buses should be individually routed to the load point with equal length runs (or equivalent resistances). Never "daisy chain" the modules’ outputs when paralleling.

The normal remote sense implementation should NOT be used in paralleled systems because the "ballasting" effect of the negative output bus lines, which is necessary for current sharing, is canceled out. In addition, remote sensing in paralleled systems will usually cause start-up instabilities and deteriorate transient response to stepped loads. If remote sensing is absolutely necessary, a circuit for implementing it without the undesirable effects decribed above is shown in Figure 9B. For true N+1 redundant systems (described in Section 5.2) remote sensing should never be used.

Paralleling For Higher Output Currents : The circuit of Figure 9A should be used when modules are paralled for greater output current. The Modules should be selected such that their individual output voltage setpoint accuracy is within 1% of one another. Up to four modules can be paralleled with this scheme with reasonably good load sharing, especially at higher output currents. The voltage adjust potentiometer and the fixed resistor across the remote sense terminals should have the value recommended in the table in section 3.6. Additional output filtering capacity should be located at the common output junction point.

The DC input to the modules can be connected in a "daisy chain" manner with each module having its own fuse and input capacitor (see Section 2.2) located close to the input pins.

For enhanced load sharing the PAR and ON/OFF (inhibit) pins should be connected from module to module as shown in Figure 9A.

For high current paralleled systems where remote sensing must be used, the active remote sense amplifier circuit shown in Figure 9B is recommended. This circuit will allow accurate remote sensing without compromising load sharing. With respect to the paralleled system’s output voltage, the input sense divider comprised of R1 and R2 should be scaled so as to always produce 2.5 volts at the wiper of VR1 (when adjusted to its midpoint). This potentiometer can then be used to trim the system’s output voltage. This circuit works by driving the parallel connected TRIM pins of the modules. The normal remote sense (RS) pins on the modules should be left disconnected when this circuit is used.

N +1 Redundant Systems :

A true N+1 redundant system employs parallel connected power modules such that :

1. The total number of modules is one greater than that required for full load.
2. The power modules share the load.
3. Failure of any one module will not affect the other modules (and full load power will still be available).

 

The system configuration must take into account all possible failure modes and assure continued operation of the system should any one module fail. Three failure scenarios are possible :

1. The output of the module short circuits.
2. The module’s input short circuits (failed inverter)
3.The module fails non-catastrophically with zero output.

Figure 10 shows a three-module N+1 redundant system which will satisfy the above criteria. Diodes are used to OR the input and output buses. The output ORing diodes should be low voltage, low forward drop Schottky types. Diode ratings with reverse blocking voltage and a current rating of at least 25% higher than the module’s current rating are recommended. These diodes will prevent a shorted module will clear its input fuse and essentially "remove" itself from the system.

Output voltage trimming of each module in an N+1 redundant system can be accomplished with the aid of the inhibit (on/off) feature and a small system "dummy" load which can be powered by any single module. With this arrangement the output voltage on each individual module can be precisely set to the desired system value by inhibiting the other modules. Once calibrated, the dummy load can be removed and the modules will share currents when the system to be powered is connected.

Although the active remote sense circuit and the PAR and ON/OFF pin connections of Figure 9B could be utilized for improved load regulation and current sharing, the true N+1 redundant concept will be seriously compromised. A failure of the active remote sense amp could drive the modules’ TRIM terminals to a lower or higher voltage than desired (+ or – 10% max.) Also an internal module failure could modify the PAR or ON/OFF interconnection lines which would affect the other modules’ outputs.

REFERENCES  

• "High Frequency Switching Power Supplies, Theory and Design", 2^nd Edition : George Chyrssis; McGraw-Hill publishing Co.
• "Switchmode Power Supply Handbook"; Keith Billings; McGraw-Hill Publishing Co.,
• "Switching Power Supply Design"; Abraham Pressman; McGraw-Hill Publishing Co.,
• "Power Line Filter Design for Switched Mode Power Supplies"; Mark J. Nave; Van Nostrand Reinhold.

PCB Drilling Details

Module Mounting Details

Module Heatsink Details

ORDERING INFORMATION :

Example 1 : Model number for 48 volt input, 5 volt output Module = HD 48-5

Example 2 : Model number for 300 volt input, 5 volt, + & -12 volt triple output Module = HD 300-5/12/12

For single output voltages and triple output configurations other than those shown in the table above, please consult the factory.