Electronics > Beginners

MC34063 high voltage dc-dc boost converter

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MagicSmoker:

--- Quote from: dazz on December 31, 2019, 08:05:44 pm ---
--- Quote from: MagicSmoker on December 31, 2019, 12:28:02 pm ---These ICs - and any with an internal switch, really - also aren't the best choice for transformer isolated converters

--- End quote ---

Can I ask you why is that the case? Aren't this IC's meant to control the switching? Or do you mean IC's with an internal switching mosfet? Sorry, I'm afraid I'm missing something here

--- End quote ---

I was referring to regulator ICs - that is, a controller + internal switch - vs. controller ICs, which require an external switch. Regulator ICs are usually designed for non-isolated applications and, as a result, often only have a single pin for feedback. Controller ICs - which control an external switch, hence the name - typically bring out one or both error amplifier inputs as well as the output which makes it easier to design a compensation network and use an optocoupler for isolating the feedback pathway.

Another possible issue with regulator ICs is that is that any ringing or voltage overshoot from the transformer leakage inductance can disrupt the controller's operation (or even destroy it) by causing substrate currents to flow. Basically, once you need to handle more than 10-20W or so you should strongly consider using an external switch.

dazz:
OK, thanks. Frequency compensation is on my list of things to investigate. In a quick google search I learned it's about avoiding positive feedback/oscillations.
Just to be clear, when you say I need an external switch, do you mean the mosfet? I guess that's not it

dazz:
Oh, OK. I think I see what you mean now. The internal switches are not necessarily mosfets. So for example in the MC34063 the internal switch is that darlington pair at the SWC & SWE outputs, right?

MagicSmoker:

--- Quote from: dazz on December 31, 2019, 08:45:46 pm ---Oh, OK. I think I see what you mean now. The internal switches are not necessarily mosfets. So for example in the MC34063 the internal switch is that darlington pair at the SWC & SWE outputs, right?

--- End quote ---

Right, the switch in a switching power supply can be a MOSFET, BJT, SCR... heck, even a relay (see the "vibrator" type power supply used in early car radios). In this case, you are mostly correct - the actual switch in the MC34063 is Q1 while the driver for it is Q2. Note that the collector for driver Q2 is brought out to a separate pin to allow for more flexibility in driving Q1; most of the time you'll just connect the collectors together in a straight Darlington configuration, though.

T3sl4co1l:
Some of those old controllers were also useful (to a limited extent) alone, so could be considered regulators as well.  34063 is kinda like that, depending on how much of the output transistors you use.  TL494 has 200mA, 40V "uncommitted transistor" outputs, so can potentially deliver a couple watts by itself.

Or, the gate drivers often capable of several amperes (if in pulses), but the always-on output rather constrains direct application of them (you can't very well run a boost converter when there's a transistor firmly clamping the pin to VCC).

Tim

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