IR2153 smps with short circuit protection.

KX36

New member
(Not looking at a schematic and assuming I know what resistor you're talking about...)
The MOSFET gate resistor damps resonance between the parasitic gate capacitance and source inductance of the FET. It's a standard component related to FETs in any application. 10-20 ohms is a typical minimum and in high speed switching, you need the minimum. Some FETs may need more than 10 ohms though. Without the resistor the FET will typically transition on and off many times when asked to transition once, greatly reducing the effectiveness of the circuit and greatly increasing the switching loss in the transistor.

If you want to protect the IC's gate driving pin from the discharging FET gate pulling it below ground voltage, you'd do that with a reverse biased schottky from the pin to ground, as close to the pin as possible. You can also have one to clamp the pin to no more than Vcc although that tends to be less of an issue.
 
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hanair

New member
Thank you KX36! Very informative post.
Just to clarify: the parasitic inductance along with MOSFET gate capacitance can resonate at frequencies like 20MHz or so, am I correct? (for mosfets similar to IRF740).

Thanks!
 

demykiko

Member
parasatic inductance whether it resonates or not with gate capacitance is not an issue here in my opinion..
because smps does not deal with that high frequency (20Mhz) unless you are dealing with RF circuit..

Regards,
demykiko
 

KX36

New member
Yes, the resonance is generally well into the MHz and yes it does matter because the ringing may continue through a significant portion of the switching period during which time the losses in the transistor may be very high making damage a significant possibility. The excess high frequency noise can also be a problem for other circuits and devices nearby and especially if you have to pass EMC.
 

demykiko

Member
show to us the actuality of the resonance you been discussing here by posting a working smps in the 20 Mhz range..
I believe everyone is aware that there is losses especially if you use a wrong choice of fet..and it must be RF fet,
suitable for such application...One of hardest circuits to treat is the RF circuit..because of stray capacitance and inductance
beyond the control of DIY'ers..and even the formula can not compensate for the existence of those strange strays..

Regards,
demykiko
 

kinter

New member
some picture of my pcb....
is almost finished but there is not IR2153
DSC_0315.jpgDSC_0316.jpgDSC_0317.jpgDSC_0317.jpg
 

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borysgo2

New member
I just was wondering what kind of advantage is to use current sense trafo ?
Can I use simpe circuit as bellow as current sense ?

I do not know If my calculation is correct.

For 3A peak to peak current I have 1.5A rms current over primary winding.
1.5A*approx 300V gives me = 450W rms --> is that calculation ok ?


Thanks
 

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borysgo2

New member
I have made some small modyfication, transistors are ''thyrystor'' connected. Instead the mcr100 can be used (I think it is better option), but it can be done with two small transistors too.
The 10k pot is used for adjustment the over-load protection. I hope it will work ok.
 

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borysgo2

New member
I have small question - do I really need inducturs after diodes ? (I saw many projects that they do not have them)

Thanks
 

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KX36

New member
borysgo2, there are advantages and disadvantages of current sense transformer. It would be more efficient than a sense resistor directly in the high current path, but like all transformers there's a risk of saturation if the current through it is unidirctional and it isn't allowed to reset with some deadtime (therefore if your entire overcurrent protection relies on one it has to be designed right), whereas a sense resistor can handle a continous DC current.

In case of an IR2153 driven half bridge like this, saturation's not much of a problem because the flux through the primary alternates in direction, so flux shouldn't build up to saturate it, although it's still best to have some deadtime to allow for imbalance. The most significant thing with a half bridge circuit though is that the lower transistor is only conducting every other cycle, so a sense resistor on the source pin of this as in your schematic will not measure the current on the other cycle, therefore a current transformer is the only real option.

As to your question about the output inductor, a proper regulated power supply based on a buck-derived topology will need one (although you might see some amateur schematics which don't have one from people who don't understand how they work and have assumed a diode capacitor secondary like in a flyback or linear supply will work). This SMPS however is unregulated; it's essentially a normal linear supply (with the minimal line and load regulation that implies) running on an internally generated high frequency instead of the 50/60Hz mains frequency in order to get more power from a smaller transformer, so the secondary doesn't necessrily need an inductor.
 

kinter

New member
borysgo2
the two resistor that supply ic should be in parallel, and the value is around 82-100kohm 2W, so the equivalent resistor is around 40-50kohm 4w.
in my pcb i have 2x 68kohm and they are very hot!

((315-15)^2)/R=2.65 watts
 

KX36

New member
The resistors kinter describes should also be rated for the full voltage expected to pass through them. It's common when there's over 200 or 250V through a resistor to split it into 2 series resistors but bigger power resistors may be rated for a high enough voltage. The more common way of powering a primary switching IC in off-line applications is with an aux winding off the transformer and a relatively high resistance resistor to slowly charge the capacitor before the switching IC starts without dissipating too much power. If you look at Norazmi's original schematic on page 1, that's what they've done. I haven't read every post in between to know if there's a reason people have deviated from this though. Resistors shouldn't be getting "very hot".

Something else I noticed in the PCB layouts here, there is insufficient creepage distance between traces on the primary and secondary. There's a significant safety concern here even if you're not worried about passing compliance tests etc. There is the potential that there could be a failure which makes the secondary side live which is dangerous if there is any physical contact between the secondary and the device chassis or secondary side circuitry and the user (switches and pots on an amplifier for example)
 

kinter

New member
hi kx36,
my project is a test for supply small amplifier 2x50watts. i needed a power supply very small and low cost! (i realized with less than 10 €)
and very simple without rewound the atx transformer.

dimension are only 100x80 mm height 38mm
 

borysgo2

New member
OK
I am alive so it means that SMPS is working.
As I have none experience with this kind of supply I will need some advice. Bellow some measurements.


TRAFO eer 35/20/11 (1,1cm^2 core):

-primary 2x17 turns
-secondary 2x11 turns (+-46V)
-secondary aux 2x4 turns

The mosfet radiator is 37 deg (i do not know if it is ok or not). The LO mosfet has higher temp than HI mosfet, LO mosfet is around 55 deg and is heating up the small radiator to 37 deg (no load). The LO mosfet --> if you take a look at the gate voltage it looks like the OFF time is longer than ON time - is the heating issue caused by the OFF time ? . The radiator with diodes is ice cold.

The snubber is 1nF--100R, gate resistors are 22R (lower them to 10R ?). The idlle consumption current is 50mA (AC).

I havend done any test with load yet.

If someone can take a look and tell me if it is looking right. May be can someone tell me what kind of tests more I should perform too (will be glad for advice )

Thanks !!!
 

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KX36

New member
At a glance, not looking bad so far. If you want to measure the high side MOSFET Vgs and Vds on the scope to compare to the low side without a differential probe, you can use 2 channels in A-B mode.
 

borysgo2

New member
I have some update. I have reduced snubber cap to 470pF and the temperatures are (ambient temp 19 deg C):
- mosfets radiator 31
- trafo 30
- diodes 19

The iddle current is 35mA (ac) - is that ok ?

I have prepared new layout cause I do not like heat produced from clamp resistors. I decided to do the same as in the first post - add auxilary winding for self supply. The GND return from ir2153 is separated form high current path - is that ok ?

And another question, bellow is primary square with no load, I have marked the sloped parts of the falling and rising edges, can I read from here If trafo primary winding is correctly winded --> the slopped part ?

Thanks and sory if my questions are bit silly.
 

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MicrosiM

Administrator
Staff member
I have some update. I have reduced snubber cap to 470pF and the temperatures are (ambient temp 19 deg C):
- mosfets radiator 31
- trafo 30
- diodes 19

The iddle current is 35mA (ac) - is that ok ?

I have prepared new layout cause I do not like heat produced from clamp resistors. I decided to do the same as in the first post - add auxilary winding for self supply. The GND return from ir2153 is separated form high current path - is that ok ?

And another question, bellow is primary square with no load, I have marked the sloped parts of the falling and rising edges, can I read from here If trafo primary winding is correctly winded --> the slopped part ?

Thanks and sory if my questions are bit silly.


1- remove your snubber, and see if temperature will get worse or better.

2- Waves looks OK, don't worry

3- Temperature ranges are good also

4- yes you MUST include the AUX winding to feed the IR2153D after it starts from the clamp resistor, once that happens, the clamp resistor will not heat any more.
 

borysgo2

New member
Thanks MicrosiM

I have removed the snoober.
The idlle current dropped to 30mA, the mosfets are ice cold now. The waves have changed now and are more symetric. The current spikes at the current sense trafo are lower too.

So I am going in right direction anyway.

Now I have to sort out the over load protection. It is not working correct now, I have to find my fault somwhere, on the current sense trafo the high spikec are causing the SMPS to oscillate at the high load. I have soldered the BC550 transistor instead MCR100 and SMPS has turned off ok. I have to chec it again and maybe add some RC to smooth it a bit.
 

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