Analysis of an RCD Clamp Circuit - Page 1
Forum section: electronics / projects

author: Yamin
author_rank: Frequent Contributor
timestamp: August 02, 2021, 04:55:19 pm
content: Hi guys, I'm going through application note for TOPSwitch-HX Family SMPS IC. I'm trying to figure out how the component values for the clamping circuit are calculated and how the output voltage is set. On page 13 its said that the voltage was limited to 600V but doesn't show any calculation.I believe the main clamp circuit components are D5,R6 and C6. I've been looking online for clamp circuit examples and first of all though there are similarities I'm not 100% sure that the examples I came across are configured as the same manner as the one in the application note. (The diode is not parallel to the resistor).So I hope someone can shed some light on this.I have attached screenshot of the clamp circuit from the application note, and attached photo of the examples I found online.Application note:https://www.power.com/sites/default/files/product-docs/an43.pdf

author: Andy Watson
author_rank: Super Contributor
timestamp: August 02, 2021, 05:22:16 pm
content: I would consider the energy in the magnetic field of the transformer. Under worst-case assumptions the transformer behaves as an inductor, so \$E = \frac{1}{2} L I^2\$, where \$L\$ is the inductance of the coil and \$I\$ is the peak-current reached during one cycle. Assume that when the current is turned-off, all this energy is dumped into the capacitor, C6. \$E = \frac{1}{2}C V^2\$. Make \$C\$ big enough to limit the voltage. The resistance, R6, must be able to dissipate most of that energy before the next cycle.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 02, 2021, 05:47:41 pm
content: Note that the above calculation is true, using the energy left over after the secondary has "caught" the flyback -- in other words, the energy stored in the primary referred leakage inductance.Which is charged to the same current as the magnetizing inductance, at switch turn-off, but the inductance is a small fraction, say a few uH (check the transformer datasheet, or measure it if you don't know -- this is the primary inductance with all other windings short-circuited). So the RCD components don't need to be very powerful, at least assuming a reasonable transformer design.The parallel R and C I think aren't needed, or their presence varies between drawings/applications. The zener/TVS does the job, as long as it can handle the power, and does so at a stable voltage drop. Whereas the RC case, voltage varies with load, affecting efficiency -- the voltage is never less than the reflected output voltage, so at light enough load, switch current must be at least enough to overcome that loss. The series resistor I haven't seen very often, but its value is small enough it shouldn't do much to conduction loss (manifest as excess switch peak voltage), meanwhile it significantly reduces EMI by damping the diode capacitance. You may also see a ferrite bead (on the diode, or as a separate chip or leaded component) to the same end (adds ESR at ~VHF). (Both diodes are necessary (the rectifier and TVS), because a unidirectional TVS alone of course won't do, and a bidirectional may conduct during the on-phase, at high enough input voltage; but the rectifier also has much less capacitance than the TVS, which would otherwise affect switching.)There's also the dV/dt (rate) snubber, arranged slightly differently and using a much smaller RC time constant (and no TVS); I don't think this is used much, or very important, for a small regulator like this, but it's an option where the load current is generally high (so that the rate is nominal; it's proportional to peak switch current, of course), leakage inductance is relatively low (since a relatively large RC is needed to absorb the peak overshoot of larger LL), and the effect is to reduce turn-off switching losses (which can increase overall efficiency a little bit). Its value can also be dimensioned to dampen free ring-down (in DCM converters), further helping EMI.Tim

author: Yamin
author_rank: Frequent Contributor
timestamp: August 03, 2021, 07:31:33 pm
content: Thanks so much guys for the info. I was also hoping that you could direct me towards more reading/research materials on this subject too.Also I would like ask since I am realising that I am not 100% understanding the reason why a 'high' voltage could be at the drain pin of the IC during switching and hence the need for the clamping circuit and the TVS diode. I do apologise for the noob question.Again really do appreciate the help