OUTPUT TRANSISTOR SATURATES ON INCREASING PWM--BOOST CONVERTER - Page 1
Forum section: electronics / projects

author: Achu
author_rank: Contributor
timestamp: May 22, 2019, 03:22:15 pm
content: Hi,I've been playing with boost converters based on TL494 and also arduino.But when I increase the pwm over 40% the output Mosfet stops switching and saturates drawing heavy current and I burned three of them already.I have also tried this on bjt with same effect.I used irf540,irf840 and also mje13007 bjt.Same thing happens on arduino based circuit.I hooked the pwm directly to the gave via a resistance.The gate is of course pulled down by a resistance.I'm no expert on switching power supplies.help me out guys,its driving me crazy.

author: Buriedcode
author_rank: Super Contributor
timestamp: May 22, 2019, 04:11:53 pm
content: With a boost converter, the on time (low side switch) determines the peak current in the inductor. The longer the on time, the higher the peak current. Inductors saturate at a certain current, and depending on the type of core, this can be gradual or sudden - this means the current will ramp up linearly, then suddenly increase.Saturation of the inductor isn't desired, so you must either 1) increase the inductance - which slows down the current ramp so the peak current is lower for a given period. 2) Increase the frequency - reducing the switching period which will reduce the on time - or 3) use a large core where the saturation current is higher for a given inductance.Put a scope on a low value/current sense resistor in series with the inductor and slowly increase the duty cycle - you will see the current rise linearly, increasing as you increase the duty.Altohugh I probably shouldn't link to other forums, this post has some valuable images that demonstrate this:https://electronics.stackexchange.com/questions/83072/calculating-the-saturation-current-of-an-inductor

author: Achu
author_rank: Contributor
timestamp: May 22, 2019, 04:33:13 pm
content: Could it be because of the lack of a gate drive circuit.Could the persisting gate charge keep transistor turned on?,beacause when we increase pwm width the gate does not have enough time to remove the charge.Also I'm using an inductor from SMPS.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 22, 2019, 05:27:02 pm
content: You need a current sensing circuit and pulse-by-pulse limiting. Usually a shunt resistor between the MOSFET source and GND is used.Without it, it's hard to get working reliably.

author: DannyTheGhost
author_rank: Contributor
timestamp: May 22, 2019, 05:29:38 pm
content: Either way, your inductor enters into saturation modeIf you could bring more details about your design: output power, what exactly inductor you are using etc., maybe we will have more chances to help youIf you have small-power design, the probability to get Miller effect (MOSFET latching 'on' during closing period) is close to none

author: soldar
author_rank: Super Contributor
timestamp: May 22, 2019, 06:35:57 pm
content: Designing a SMPS is not trivial and the inductor needs to be designed very carefully.Quote

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2019, 08:16:57 pm
content: Post a circuit.Tim

author: Achu
author_rank: Contributor
timestamp: May 23, 2019, 02:33:06 am
content: Here is the circuit.I intend the converter to produce 30V rated output and a maximum of 55V at a current of 2A.The inductor I used was a rewound torroidal one from the filter of ATX. It had a value of about 200uH.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2019, 03:11:07 am
content: Are you sure that's drawn correctly? It doesn't look like it'll do anything at all.Tim

author: Achu
author_rank: Contributor
timestamp: May 23, 2019, 03:38:20 am
content: There is a mistake actually the error amps are parallelled

author: DannyTheGhost
author_rank: Contributor
timestamp: May 23, 2019, 06:44:36 am
content: The circuit you've drawn is actually making no senseThis actually doesn't even look like any dc-dc topology we know so farFirstly, TL494 has 2 error amps connected in OR configuration, so you can only use only oneSecondly, boost dc-dc topology means 'inductor getting shorted to ground by switch and when switch is disconnected inductor adds its back EMF voltage to VCC, but what you have here on your circuit is 'your output is getting connected to input voltage via inductor and diode'

author: mikerj
author_rank: Super Contributor
timestamp: May 23, 2019, 07:00:13 am
content: Quote from: Achu on May 23, 2019, 02:33:06 amThe schematic is clearly incorrect, but what is your input voltage? This will determine switch current for a given output. 55v and 2Amps is 110 watts, not a trivial amount of power.Note that a UF4007 diode is only rated for 1 amp.

author: Achu
author_rank: Contributor
timestamp: May 23, 2019, 12:42:51 pm
content: Sorry I drew the circuit in a hurry.And I have modified it.Please check the initial post.This is my first easy EDA dawing.Supply voltage is 12V.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2019, 03:17:24 pm
content: Can barely make out the labels at that scale.The boost section looks to be wired like a boost section.It still doesn't look like it should do anything but blow transistors, though. The "E" pin is pull-up only, there is no way to pull down an already-pulled-up transistor gate. (I forget how much current goes into the pin; it's not exactly an emitter, there is some internal circuitry attached. I don't think it can pull down this much, though.)The missing feature would seem to be that you need a gate driver.You also need a current sense, and compensation for feedback. The current sense is key, as that prevents the transistors from exploding. Everything else is secondary.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2019, 03:24:59 pm
content: You may find this is more practical,(the top boost section, don't mind the bottom inverter section). This uses a peak current mode controller,There are some typos on this exact schematic; the controller should be UC3843 (starts up at 5V supply, not 16), the 20uH inductor needs to be low loss (a "drum" style ferrite choke will do), and some slope compensation would be nice, or a lower value inductor can be used.To save power, the current is sensed by a current transformer. An off-the-shelf 1:100 CT can be used, in which case the 2.2 ohm resistor value drops a little for the same ratings.To get more output voltage, the feedback divider ratio changes of course, and the transistor and diode ratings need to increase (use 80 or 100V parts for a 55V output).Note the SB540 and 1uF capacitor across the transistor: these must be placed as close as possible, so that at the moment the transistor turns off, its load current is transferred to the capacitor, and stray inductances discharge into it. This isn't needed if the SBL2040 diode and output capacitors are equally nearby, but that may be hard to arrange, in which case having the smaller diode and capacitor up close are effective at protecting the transistor.The 3842 family is easier to use than the TL494, I would say. It's also safer to use -- because of the current mode operation.Tim