Dissipation in SMPS drive circuit very high?

[opampsmoker]

High
Page 13 of the MC33063A datasheet shows a 180R resistor in the collector of the drive transistor.
It will be dissipating 624mW when the FET is ON, which for high duty cycles, will be most of the time.

Why does the MC33063A circuit here even exist?......thats far too much dissipation in a drive circuit.

MC33063A datasheet

https://www.ti.com/lit/ds/symlink/mc34063a.pdf?ts=1605032359878&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FMC34063A

 

[ronsimpson]

Two circuits below. One is Darlington where the voltage from Collector-1 to Emitter-2 is high (when Q1 is on) so there is power lost in Q1. In the other circuit power is lost in the 180 ohm resistor. The C-E voltage when Q1 is on is much lower. You have a choice.

Another option: Left side of inductor is just below 12V. The right side is 0.3 to 0.5V. I tapped off the inductor at about 1 to 2 volts. Reduced the resistor to 10 ohms. Now Q2 has the same current but the resistor has 1/6 the voltage and 1/6 the power lost.

 

[opampsmoker]

Thanks,
VCE(SAT) for the Darlington connection is given on page 6 of the MC33063A datasheet..

MC33063A datasheet

https://www.ti.com/lit/ds/symlink/mc34063a.pdf?ts=1605032359878&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FMC34063A

....and is only 1V at 1A, so this doent seem too high for us.
Would you agree page 6 is applicable to Darlington.?....it does seem very low VCE(SAT) voltage for a Darlington? A little too low?

 

[ronsimpson]

The VCEsat on page 6 looks like 0.65 volts more in Darlington mode which is just right.

The extra power loss in Darlington mode is almost exactly what is lost in the resistor. (at 1A)

 

[opampsmoker]

Please may i also ask this question about how to calculate the switchign frequency in the MC33063A?..

Hi
The MC33063A PWM controller has an oscillator frequency described by page 6 of its datasheet….this is a graph showing FET ON time and FET OFF time.
For a 1nF CT capacitor, this means 20us ON time and 4.5us OFF time….

That’s a period of 24.5us, which gives a frequency of 40800Hz..

..But what happens if the current sense trips the FET OFF after just 5us of ON time?……………does the MC33063A then immediately start discharging the timing capacitor, and thus give a much higher switching frequency than 40800Hz?

Page 4 of AN920 does suggest that when the current sense comparator is tripped, one will see a higher switchign frequency, and a longer OFF time.
AN920

https://www.onsemi.com/pub/Collateral/AN920-D.PDF

.....would you agree with this?

MC33063A datasheet:

https://www.ti.com/lit/ds/symlink/mc33063a.pdf?ts=1605102742932&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FMC33063A%253Futm_source%253Dgoogle%2526utm_medium%253Dcpc%2526utm_campaign%253Dapp-null-null-GPN_EN-cpc-pf-google-sa%2526utm_content%253DMC33063A%2526ds_k%253DMC33063A%2526DCM%253Dyes%2526gclid%253DCjwKCAiAtK79BRAIEiwA4OskBrcB6T9fYYcz9LCaOZgsRhMZJdzGUQsjsP8gHGi9Azf_LdjBjmnPFBoCgAwQAvD_BwE%2526gclsrc%253Daw.ds