1K is pretty huge for a current sense resistor. Though the idea of bypassing with parallel capacitors might lower its loss a bit, it will still conduct all the DC.Hi,
Tie the negative end of the lower bulk capacitor and source of the low side MOSFET together. Connect a resistor between this point and the negative point obtained from the rectified mains. Use a 1k resistor, connecting one end to the joined point mentioned above and a 0.1uF and 10uF cap in parallel between the other end of the resistor and ground. Use an op-amp to amplify the voltage and use a comparator to compare the voltage across the resistor with a set voltage (that for the maximum load). When the voltage across the resistor is higher, the comparator can turn on an SCR that shuts down the controller (SG3525, TL494, etc). Since an SCR is used, you need to reset the power to turn the converter on again.
I haven't tried this, but I think this might work.
Hope this helps.
Tahmid.
...all it would need would be a pwm controller with current sense shut-off (i.e. FET off when overcurrent occurs as opposed to current mode control)
Ah okay, the 1k is just for a low pass filter, I misunderstood.I didn't mean that the 1k should be the sense resistor. The sense resistor could be 1R, 0.1R or even 0.01R. 1k and capacitors are to filter spikes.
Hope this helps.
Tahmid.
No it's not in the primary current flow. And it's not as fast as being in line with one or both of the switches, or in series with the primary. I can guarantee that if you get a short on the output, the input will not react within on switching period. Or probably several, for that matter. Whether it acts before the supply destroys itself is unknown. Depends on the characteristics of the transformer, and the FETs. Anyways, you just need to determine what response speed you need. 100us, 1ms, 10ms... all are useful for some faults but not for all faults.A sense resistor placed in the way that Tahmid kindly described, would be the fastest, safest and best way of quickly detecting primary side overcurrents in the half bridge.
....that is obvious because it is right in the primary current flow.
Yes, Orson has commented in post #14 about the disadvantages. Reviewing the original post, no clear purpose for the current measurement has been given yet. Without it, all assumptions about needing a fast measurement, preferably applied to both outputs transistors are void in part. I agree with Orson, that overcurrent or short circuit protection would require to measure the instantaneous output for both current pathes, and I would tend to assume it as a standard. Otherwise, we can ask why current is measured at all.Just to clarify, this is what I meant.
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