I know the definition of Hold-up time in SMPS, and how to calculate the bulk capacitor. But I am wondering if someone can explain me how we define the hold-up time (sometimes 10ms, sometimes 16ms,…)? is it based on a standard?
and why we need the hold-up time? what happen if we dont have it
Electronic – Hold-up time in SMPS
switch-mode-power-supply
Related Solutions
Capacitor choice is largely a matter of losses, yes. The capacitance value needs to be large enough to keep the midpoint more-or-less-fixed, with low ESR and ESL also being important. Under some transient conditions the midpoint can (and will) move around, so it's important to keep derating in mind as well. 50% voltage derating is well within reason (using 400V caps with a total DC input of 400V, for instance).
For AC-application capacitors (i.e. film caps), the ESR is often implied in the dissipation factor datasheet parameter, which is the relationship between ESR (resistive loss) and reactance:
\$ DF = \omega C \cdot ESR = \dfrac{1}{Q} \$
\$ \omega = 2 \pi f \$
It will usually be specified at a given frequency, which allows you to calculate the ESR.
Electrolytic capacitors generally have large ESL by nature of their construction vs. film capacitors, limiting their usefulness at high frequency. Film capacitors also have some self-healing capability under dielectric breakdown scenarios which make them a 'robust' choice in half-bridge designs.
No a PWM controller is not a good fit for this application because it has not regulation capability, it has no feed back. In a true buck regulator, the regulator senses the output and adjust the PWM accordingly to make sure that the output is the correct voltage. This kind of regulation compensates the system across many different loads. If the PWM is fixed then it is only valid for a single load point.
A prebuilt buck regulator is the easiest solution for you. Add and inductor and some passives and your up and running. If you really want to roll your own there is a topology that uses two opamps and a triangle wave generator.
simulate this circuit – Schematic created using CircuitLab
These are not the correct values. I will leave it up to you to figure them out if you choose to use this setup.
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Best Answer
There are standards that indirectly define hold-up times.
The standard IEC 61000-4-11 Voltage dips, short interruptions and voltage variations immunity tests indirectly defines hold-up time when the product has to satisfy performance criteria A.
This document shows some tests (e.g. voltage dip of 40% of 20 ms duration or voltage interruption of 60 ms duration).
For medical devices, the standard IEC 60601-1 applies.
See this document for some durations of voltage dips and interrupts.