stenzer
Advanced Member level 3
Hi,
I'm aiming to design a push-pull converter for isolation purposes. Therefor, I intend to use a primary and secundary center tapped transformer, which enables a full wave rectification with two diods (circuitry shown in [1]). For simplicity I'm neglecting the diode voltage drop as well as the voltage drop at the switching MOSFET at this point, thus assuming a transformer turn ratio of Ns/Np = 1 (I'm only interested in the isolation).
My design parameters are:
Vin = 5 V (regulated; MOSFET voltage drop is neglected)
Vout = 5 V (diode voltage drop is neglected)
Pout,max = 5W
f_switch = 250 kHz
D = 50 % (duty cycle)
The designed converter should be used for a low product quantity (~100 pieces), thus an off the shelf transformer should be used, and that's the point where I'm start to struggle.
A couple of datasheets of PWM controllers explain how to choose a suitable transformer by means of the ET product (sometimes also called VT product). Which accomplished by choosing a transforme which fullfills
ET >= Vin / (2 * f_switch) = 5 V / (2 * 250E3 Hz) = 10 V*µs
So if I'm using a transformer which has an ET larger (or equal) than 10 V*µs everything should work fine for me. E.g. [2] (has a turns ratio of 1:1.1) with an ET product of 27 V*µs. To my knowledge for a center tapped application this value has to be divided by 2 (as at each switching period only the half primary inductance is applied to Vin).
Although the ET product method seems to be a fast and easy way to select a transformer I have the feeling I'm missing something i.e. transformer current rating (interestingly [2] does not include a current rating) and power. I know the magnetic flux B is proportional to Vin * ton [3, page 14]. Further, B = µ0 * µr * H, where the magnetic field strength H is a function of the coil geometry i.e. number of coil turns, length and diameter as well as current. So the current rating might be covered by the ET product, but it is not obious for me.
Further, there are also off the shelf transformers available, which are not stating the ET product e.g. the PA3964.002NL [4]. The voltage rating and turn ratio of [4] do no fit with my design considerations, this reference is only given to show there are transformers without stating an ET product.
So my overall questions are:
- If I'm finding a transformer which has an ET product larger than that calculated above, I'm fine? (what't about the power and current?)
- How can I identify if a fransformer can be used, not stating an ET product?
Maybe someone can resolve my uncertainties. Unfortunatly, electromagnetism is not my strength :-?.
BR & thx
[1] https://cdn.eeweb.com/articles/articles/Isolated-DC-DC-Converter_5-1429519249.png
[2] https://www.we-online.de/katalog/datasheet/750315090.pdf
[3] **broken link removed**
[4] https://products.pulseelex.com/files/product_files/P730.pdf
I'm aiming to design a push-pull converter for isolation purposes. Therefor, I intend to use a primary and secundary center tapped transformer, which enables a full wave rectification with two diods (circuitry shown in [1]). For simplicity I'm neglecting the diode voltage drop as well as the voltage drop at the switching MOSFET at this point, thus assuming a transformer turn ratio of Ns/Np = 1 (I'm only interested in the isolation).
My design parameters are:
Vin = 5 V (regulated; MOSFET voltage drop is neglected)
Vout = 5 V (diode voltage drop is neglected)
Pout,max = 5W
f_switch = 250 kHz
D = 50 % (duty cycle)
The designed converter should be used for a low product quantity (~100 pieces), thus an off the shelf transformer should be used, and that's the point where I'm start to struggle.
A couple of datasheets of PWM controllers explain how to choose a suitable transformer by means of the ET product (sometimes also called VT product). Which accomplished by choosing a transforme which fullfills
ET >= Vin / (2 * f_switch) = 5 V / (2 * 250E3 Hz) = 10 V*µs
So if I'm using a transformer which has an ET larger (or equal) than 10 V*µs everything should work fine for me. E.g. [2] (has a turns ratio of 1:1.1) with an ET product of 27 V*µs. To my knowledge for a center tapped application this value has to be divided by 2 (as at each switching period only the half primary inductance is applied to Vin).
Although the ET product method seems to be a fast and easy way to select a transformer I have the feeling I'm missing something i.e. transformer current rating (interestingly [2] does not include a current rating) and power. I know the magnetic flux B is proportional to Vin * ton [3, page 14]. Further, B = µ0 * µr * H, where the magnetic field strength H is a function of the coil geometry i.e. number of coil turns, length and diameter as well as current. So the current rating might be covered by the ET product, but it is not obious for me.
Further, there are also off the shelf transformers available, which are not stating the ET product e.g. the PA3964.002NL [4]. The voltage rating and turn ratio of [4] do no fit with my design considerations, this reference is only given to show there are transformers without stating an ET product.
So my overall questions are:
- If I'm finding a transformer which has an ET product larger than that calculated above, I'm fine? (what't about the power and current?)
- How can I identify if a fransformer can be used, not stating an ET product?
Maybe someone can resolve my uncertainties. Unfortunatly, electromagnetism is not my strength :-?.
BR & thx
[1] https://cdn.eeweb.com/articles/articles/Isolated-DC-DC-Converter_5-1429519249.png
[2] https://www.we-online.de/katalog/datasheet/750315090.pdf
[3] **broken link removed**
[4] https://products.pulseelex.com/files/product_files/P730.pdf