Electronic – What’s a safe method for determining the switching frequency of a blackbox ACDC converter

The Q and M curves show that the resonant frequency is a property of the transformer, the load and the series capacitor. \$f_o\$ isn't necessarily a 'shorted' resonant frequency but a mostly-load-independent second resonant point predominantly controlled by the leakage inductance and series capacitor.

The switching frequency is indeed controlled by the some form of control IC (usually a PWM). The frequency of operation will vary as a function of input line and load, and should stay above the worst-case (minimum load) resonant peak to ensure ZVS over the whole range. You are correct in that operation below the worst-case peak gain forces the converter into an uncontrolled region and should be avoided.

LCD TV and laptop adapters are usually < 24V. If the designer was able to use a schottky diode for the output rectifier, you don't have to rely on forced commutation from the primary to reduce losses. The best way to tell if you're suffering from excessive reverse recovery loss is to put a current probe in the secondary and measure the reverse recovery current. Operating this 70V converter above resonance means they're not soft-commutating the secondary (which isn't necessarily bad) to reduce conduction losses (less circulating current in the primary).

I think that Cypress have invested a fair amount of money in developing this chip and the first thing i'd do is try and get reassurance from them about your concerns. Obvious (or not) PCB layout will be key to avoiding noise problems on your analogue circuits so follow the guidelines they give on PSU layouts in the PDF.

From my standpoint, I've recently developed a circuit that has 16 strain gauge inputs, provides DC constant current bias for them, amplifes and filters them, converts to digital, combines the ADC streams and applies "comms" header to give a single 500kbps serial stream and transmits it to a receiver via a local FM transmitter. It also has a switching regulator on board and I know there is no noise I can measure that is attributable to it.

This design has good low noise performance (2mVp-p is typical input signal) and the noise performance would have been a lot worse should we not have used good PCB layout practises.

It boils down to: -

1) Do you trust a reputable chip supplier to sell a device that is compromized

2) Why not seek assurances

3) Circuits that require much higher noise performance work with SMPSUs

4) Layout, good layout.