SN6501 Isolation Transformers for 15V

Hi Lukas,

There is no reason you cannot step the voltage up on the secondary side of the transformer. The only issue will be for you is that you will be trading off on current driving capabilities. The device is only capable of driving a continuous power of 250mW (so at a 5V VIN you are limited to 50mA continuous). Therefore, you will not be able to generate a 50mA continuous supply at 15 volts on the secondary with this device.

If your current requirement can be lowered, there are multiple topologies shown on page 22 of the datasheet that can step up the output voltage of the secondary. Are you just looking for transformer part numbers? If so, I would recommend contacting any one of the transformer manufacturers that we have listed on page 22 of the datasheet.

Another option is to look at the TPS55010 which is a higher power isolated DC/DC converter with additional features.

http://www.ti.com/product/tps55010

Thanks,

John

I saw the TPS55010 but the problem is that it is a QFN which causes various problems (AOI etc.). Is it possible / recommended to boost the SN6501 output current with external FETs?

Hi Lucas,

The D1 and D2 outputs of the device are going to go from High Impedance to GND. Are you thinking that you would use this to drive a P-Channel FET? Not sure if I am following what you were thinking.

Thanks,

John

Hi John,

I think the SN6501 is capable of driving more than 50 mA continuous. 250 mW is the absolute maximum power dissipation of the device. With a 5V input, the maximum primary-side current drive is 350 mA. Ambient temperature and airflow might restrict this value. The SN6501's output transistors have a maximum on-resistance of 2 Ohms. At 350 mA, the power dissipation would be 0.35*0.35*2 = 0.245W. If the ambient air temperature is too high, the maximum Junction Temperature of 170 deg C might become the limiting factor. The data sheet for the SN6501 shows many graphs that include output currents up to 100 mA at 5V, which would be proportionately smaller if the output is 15V.

Regards,

Greg

Hi Greg,

You are correct, I made a mistake (I just took power and divided by voltage which makes the incorrect assumption that the full 5V is dropped over the low side drivers... which is incorrect). Assuming that your ambient temperature is below ~95°C, you can continuously draw 350mA of current and not exceed the 150°C max junction temperature. I apologize for the mistake.

In terms of the typical curves in the datasheets, one of the caveats is that these are only typical and do not show worst case which is what you have to design for. How much current at 15V do you need?

Thanks,

John

Hi John,

Sorry for the delay in replying, and thanks for the correction and the additional information. I hadn't performed that calculation before. It can handle quite a bit! The max junction temperature for the SN6501 is 170 degrees C, which allows it to handle a higher ambient than 95 degrees C. Given that the on-resistance of the FETs is likely going to be less than 2 Ohms (typical is 0.6 Ohms), it should be able to operate up to the maximum recommended ambient temperature of 125 degrees C. But that isn't guaranteed. Only 2 Ohms is, which limits the ambient to less than 125 degrees C.

I think you're asking Lukas the question about the 15V current requirement. It doesn't look like he is going to reply. So I can reply instead. In my application I only need about 10 mA at 15V. I also need 5V, so I am thinking of using a charge pump to get from 5 to 15. I studied the SN6501 data sheet and determined that the suggested alternate topologies in Section 9.2.4 for higher voltages that use half-wave rectification will not work. The reason is because one transistor of the SN6501 ends up feeding one capacitor, and the other transistor feeds the other. If the load is asymmetrical, there will be a core magnetization issue due to the asymmetry. So for a 15V output, I would recommend one of the full-wave bridge circuits because both transistors take turn feeding the same capacitor(s). With a 2:1 transformer, that limits the maximum to +/- 10V, or +20V. If more voltage is needed, a higher turns ratio is necessary. Beware of section 9.2.2.1 which reads "The SN6501 transformer driver is designed for low-power push-pull converters with input and output voltages in the range of 3 V to 5.5 V. While converter designs with higher output voltages are possible, care must be taken that higher turns ratios don’t lead to primary currents that exceed the SN6501 specified current limits". Hope this info helps anyone planning on using the SN6501 in a higher voltage application.

Thanks,

Greg

Hi Greg,

I agree, for typical condition the device can handle quite a bit of power. The most common feedback we get with this device is the want to handle more power. We appreciate the feedback and will use this for future devices.

In terms of the higher output voltage diagrams on page 21 of the datasheet, I agree that it is important for the schematics that charge different output caps on each output cycle of the SN6501 to have equal loading. I appreciate the detailed explanation.

Please let us know if you need anything else.

John

Hi John,

I hadn't thought about needing more power. I suppose if you make some higher-powered drivers, Wurth will make the companion transformers to go with it. I really like the engineering the two companies have done to provide a low-cost isolated power solution that works well. I won't complain about higher power as long as it is also bulletproof.

Thanks and regards,

Greg

Hi Greg,

I am hoping it will be more bullet-proof! :)

Two other things we often hear are the ability to have a soft-start and a current limit. I believe with the current limit the device will be more robust, whereas the soft start is more of a nice to have feature.

Thanks,

John