# How is the switch current related to output current in a DC-DC converter?

I have designed and implemented a DC-DC converter with the following specs:

• Input Voltage: 5V
• Input Current (max): 3A
• Output Voltage: 12V
• Output Current (max): 1A
• DC-DC Controller: MC34063
• Inductor: 15uH/5A max
• Schottkey diode, 550mV Vf 1A max
• 50KHz switching frequency

I have tied the Drive and Switch collector to form a darlington pair. The device outputs 12V, but the controller gets very hot (way beyond spec of 70C) even at 0.5A at the output.

The device says it has a maximum switch current of 1.5A, and I've incorrectly thought this is the maximum output current rating.

How do I figure out what the maximum output current will be, if I'm specified the maximum switching current?

• What caveman said | Iin x Vin x efficiency = Iout x Vout ALWAYS. | For boost as shown Vout "stands on Vin" and gives you Vin "free lunch". You need another (12-5) + Vdiuode + losses. Switch will be on about 60%-70% of time - say 60%. Vswitch avg on = I_convert/60% so 34063 falls far short of task. | You can drive an external bipolar or mosfet with it and get essentially unlimited power as far as IC ratings are concerned. Bipolar - can use IC to drive. FET - need simple cheap gate driver - ask if interested. Jan 7, 2015 at 7:53

The MC34063 datasheet actually tells you on page 11: $$I_{pk}(switch) = 2I_{out(max)}(\frac{t_{on}}{t_{off}}+1)$$

For more details, it points you to the OnSemi application note AN920A/D. I highly recommend reading these. Also, Texas Instruments has some pretty good application notes regarding switching power supply design.

As an aside, you have under specified D1, it will see currents that are higher than 1A for sure. Also, if you want good efficiency, be very careful with your layout.

• Isn't I(pk) the peak and not the RMS current? Would I have to find the average Iswitch current by using ton and toff? Jan 7, 2015 at 4:24
• You wouldn't generally use RMS current for the switch. Usually, you design according to peak current. This is why RMS isn't anywhere in the datasheet. And in general, you should design power supplies according to worst case operating conditions, i.e. max current. Jan 7, 2015 at 4:51
• Thanks. Will D1 see currents a lot higher than 1A (e.g. Ipk) or around +/-10%? Not sure how to spec this component. Jan 7, 2015 at 5:05
• That depends on the current ramp that you have. It will see currents as high as the peak switch current. The switch limits at 1.5A, so you cannot go higher than that. Jan 7, 2015 at 5:17