Managing Holdup / 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.
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 holdup 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. Here below is an example of typical Holdup circuitry:
The table hereafter 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.
| Input bus level before drop out | Holdup voltage capability |
|---|---|
| 28V (normal) | 19V (28V – 9V) |
| 16V (emergency) | 7V (16V – 9V) |
| 9V (low transient) | No holdup capability |
To help reduce size, inrush current and monitor the state of charge, an integrated high voltage module increases the voltage the holdup 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 holdup 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: a constant 38V voltage. In addition, the holdup module features an active inrush current limitation and all necessary circuitry to manage automatically the interruptions, holdup mode switching and monitoring.
The example below shows a typical example of a 25W dc-dc converter power supply requiring 200ms holdup time from a charged capacitor voltage before power drop out of 24V :
| Application requiring 200ms holdup | Cap value (µF) | Converter (cubic inches) | Typical holdup cap volume (cubic inches) | Holdup mod. volume (cubic inches) | Inrush current (A)* | Total volume (cubic inches) |
|---|---|---|---|---|---|---|
| 9-36V 25W dc-dc with basic holdup cap. | 25000 | 2 | 4.9 | 0 | 45 | 6, 9 |
| 9-36V 25W dc-dc with holdup module | 9000 | 2 | 1.6 | 0.52 | 1 | 4, 12 |
| Inrush current reduction | 98% | |||||
| Capacitor volume saved | 67% | Total volume saved | 40% | |||
On top of significantly reducing the overall function size and inrush current, reducing capacitor value increases dramatically their reliability.
As shown above, 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 holdup capacitor in order to multiply by almost 4 the stored energy for a given capacitance.
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. The table hereafter 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.
| Basic Holdup | High voltage Holdup | |
|---|---|---|
| Input bus level before drop out | Holdup voltage capability | Holdup voltage capability with 60V setting |
| 28V (normal) | 19V (28V – 9V) | 51V (60V – 9V) |
| 16V (emergency) | 7V (16V – 9V) | 51V (60V – 9V) |
| 9V (low transient) | No possible holdup | 51V (60V – 9V) |
The example below shows a typical example of a 200W dc-dc converter requiring 200ms holdup 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:
| Application requiring 200ms holdup | Cap value (mF) | Converter (cubic inches) | Typical holdup cap volume (cubic inches) | Holdup mod. volume (cubic inches) | Total volume (cubic inches) |
|---|---|---|---|---|---|
| 9-60V 200W dc-dc with basic holdup cap. 16V | 530 | 2.8 | 153 | 0 | 155.8 |
| 9-60V 200W dc-dc with holdup module | 25 | 2.8 | 4.27 | 0.85 | 7,92 |
| volume saved | 95% | ||||
For such high power applications, the amount of capacitor usually makes the holdup function not practical all together because of its 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 the following drawing:
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 module is completely transparent during normal operation and doesn’t affect system stability or EMI performance, even during holdup mode.
More details regarding such architectures can be found on Gaia Converter website, especially the application note “Modular Power Architecture Up to 300W for Avionics/Military Applications.”
1"x1.58"x0.5" package picture for the HUGD-300:
