Switch Mode Power Supplies - Primary vs Secondary Energy Storage?

Thread Starter


Joined Feb 28, 2018
I'm using a few different SMPS types for my audio amplifier designs. Some have the large storage caps before the isolation transformer and some have them after.

Primary side: 270u 400V
Secondary side: 820u 50v (one for each +/- rail)

Primary side: 47u 400V
Secondary side: 3300u 50v (one for each +/- rail)

Are there advantages or disadvantages to either design?


Joined Feb 24, 2006
There is not enough information to construct an intelligent answer. Contrary to your expectations, energy storage is probably not the main purpose of those capacitors, but without a schematic it will be difficult to say one way or another. In a flyback converter, which uses a transformer, isolation may or may not be a primary concern. Again the schematic will reveal such information.


Joined Feb 8, 2018
If you could post photos of the circuit boards it would help.

The short answer is that the two generally do not trade off against each other and different topologies place different demands on the input and output capacitors.

From your description, each supply has just one 400 volt capacitor. This implies either flyback or forward converters. Two other topologies are not ruled out but are very unlikely. If the supplies have wide input voltage range (eg 100 to 240 VAC nominal) and there is no active power factor correction, they are almost certainly flyback converters. If there are output inductors other than small "polishing" inductors they are forward converters.

Wide input voltage range without active power factor correction typically requires more capacitance on the input to keep the minimum voltage adequately high. With power factor correction less capacitance can typically be used because the high voltage is pre-regulated by the PFC boost converter.

Using output capacitance much higher than "required" is common for reasons of longevity and to cope with voltage ripple due to equivalent series resistance (ESR) of the capacitors. Flyback converters demand more of the output capacitors than forward converters. Increasing the capacitance on the primary side is sometimes done for longevity reasons and/or for better "hold up" in the event of AC power loss. Hold-up is important for certain applications where events like one or two half-cycle drops of the AC line voltage are expected or where something benefits from a few tens to hundreds of milliseconds to shut down gracefully or to permit switch-over from line power to an uninterruptible supply.

Flyback converters which operate in "continuous current mode" (which is something of a misnomer - continuous magnetic flux is more accurate - the mag flux does not return to zero each cycle) have a fundamental issue (right half-plane zero) that cannot be overcome with "frequency compensation" and makes the dynamic response fairly slow in comparison with other converters operating at the same frequency. Such converters can benefit from a comparatively large amount of output capacitance to cope with fast dynamic loads. Audio amps are moderately fast dynamic loads.