Managing hold up / transparency time in high reliability power supplies

By CHRISTIAN JONGLAS, Applications & Technical support manager, www.gaia-converter.com

As soon as power supply designers know they need to comply with military standards, they typically need to take into account the transparency, or holdup requirement of the supply. Transparency requirement is the minimum amount of time during which input power can go away but the equipment is expected to remain operational. This power interruption duration can range from 50 to 1,000 ms, according to DO160 or Mil-Std 704 standards. The simplest way to achieve such a hold-up function, is to connect to the input bus a huge tank capacitor that will store energy during normal operation, and restore it during power interruption.

The necessary capacitance value is given by the following formula :
Total Capacitance= 2 x P x Δt / (η x (V12 – V22))
where :
P = Power at the load
Δt = Hold Up time required
η = Converter efficiency
V1 = Charged capacitor voltage before power drop out
V2 = Final input voltage before power supply shut down

As an example, achieving 200-ms transparency on a 28-V input bus, a 25-W dc/dc converter with 9 to 36 V input voltage range requires a capacitor value as high as 25,000 μF / 50 V. Not only will this capacitor be very big, but it will also result in very high inrush current to charge it, unless some additional circuit comes into play to limit it. Figure 1 is an example of a typical holdup circuitry:

 Fig. 1: Typical holdup circuit

Table 1 shows the drop out voltage V1 and V2 that account for different conditions of capacitor charge voltage. For example it shows the shut-down voltage value of the dc/dc converter for normal operation, emergency operation, and low transients on the input bus, before the power drops out – down to 9 V. 

 Table 1: The drop-out output voltage for different capacitor charge voltages

To help reduce size, inrush current and monitor the state of charge, an integrated high voltage module increases the voltage the hold-up capacitor is charged to, thus increases the stored energy for a given capacitance value. Said differently, for a given needed energy, increasing the voltage the hold-up cap is charged to, reduce the needed capacitor value, and consequently the capacitor size. This hold-up module, such as GAIA Converter's HUGD-50, embeds a charger that charges the hold-up cap to the max. voltage that dc/dc converter sustains.

In addition, the hold-up module features an active inrush current limitation and all necessary circuitry to manage automatically the interruptions, hold up mode switching and monitoring. Table 2 shows a typical example of a 25-W dc/dc converter power supply requiring 200-ms hold up time from a charged capacitor voltage before power drop out of 24 V: On top of significantly reducing the overall function size and inrush current, reducing capacitor value increases dramatically their reliability.

 Table 2: Example of 25-W dc/dc converter that requires 200-ms hold-up time

As shown in Table 2, the higher the hold-up capacitor voltage, the smaller it can get for a given hold-up time. Exploiting this higher voltage principle together with ever wider input voltage ranges available with high end dc/dc converters will help reduce hold-up size even more dramatically.

Figure 2 shows the block diagram of the HUGD-300, which is not only 300-W rated but also able to deliver a user programmable 30 to 80 V voltage to the hold-up capacitor in order to multiply by almost 4 the stored energy for a given capacitance.

Fig. 2 HUGD-300 block diagram   

In addition to the charger, switching and monitoring functions, it also features a reverse polarity function to help designer to comply with several standards such as Mil-STD-704, Mil-STD-1275. This complete set of functions permits to preserve the maximum hold-up capability voltage whatever the input bus level before drop out is. Table 3 shows the drop-out voltage V1 and V2 considering different conditions of capacitor charge voltage (such as in normal operation, emergency operation or low transient input bus) before power drop out down to 9 V, the shut-down voltage value of the dc/dc converter.

 Table 3: The drop-out Voltage with different capacitor charge voltage

Table 4 shows a typical example of a 200-W dc/dc converter requiring 200-ms hold up time, using a 9 to 60-V input voltage range dc/dc, such as GAIA's 200-W series, from a charged capacitor voltage before power drops out from 16 V. 
 

 Table 4: Application requiring 200-ms hold-up time

For such high power applications, the amount of capacitor usually makes the holdup function not practical all together because of it's prohibitive size, inrush current, cost and reliability. With higher voltages and integrated switching holdup modules such as the HUGD-300 this now becomes feasible at just a fraction of all these critical parameters.