Managing hold up/transparency time in high-reliability power supplies

As soon as a power supply designer needs to comply with military standards, he might need to take into account the transparency or holdup requirement. 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 50ms, all the way to 1000ms for DO160 or Mil-Std 704 standards, for example. The simplest way to achieve such a hold up function is to connect to 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 200ms transparency on a 28V input bus, a 25W DC/DC converter with 9-36V input voltage range requires a capacitor value as high as 25 000μF / 50V. 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 shows an example of typical Holdup circuitry:

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Figure 1: An example of typical Holdup circuitry

Table 1 shows the drop out voltage V1 – V2 considering different conditions of capacitor charge voltage (such as in normal operation, emergency operation or low transient on the input bus), before power drop out, down to 9V, the shut down voltage value of the DC/DC converter.

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Figure 1: An example of typical Holdup circuitry

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 maximum voltage that DC/DC converter sustains: a constant 38V voltage.

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 25W DC/DC converter power supply requiring 200ms hold up time from a charged capacitor voltage before power drop out of 24V. On top of significantly reducing the overall function size and inrush current, reducing capacitor value increases dramatically their reliability.

As shown in Table 2, the higher the holdup capacitor voltage is, 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 holdup size even mode dramatically. The new HUGD-300 is not only 300W rated but also able to deliver a user programmable 30V to 80V voltage to the hold up capacitor in order to multiply by almost 4 the stored energy for a given capacitance.

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Table 2: A typical example of a 25W DC/DC converter power supply requiring 200ms hold up time

On top of 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- 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 9V, the shut down voltage value of the DC/DC converter.

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Table 3: The drop-out voltage V1- V2 considering different conditions of capacitor charge voltage

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

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Table 4: A typical example of a 200W DC/DC converter requiring 200ms 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. A typical architecture for Mil/Aero application is shown in Figure 2.

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Figure 2: A typical architecture for Mil/Aero application

The first 2 functions shown on the left are EMI filtering and transient protection (such as respectively Gaia's FGDS and LGDS module families), then the Holdup function comes into play, right before power is fed to the DC/DC converter(s). It is important to note that the HUGD-50 or 300 Holdup modules are completely transparent during normal operation and don't affect system stability or EMI performance, even during holdup mode.