Featuring zero voltage switching and low voltage stress, LLC resonant transformers are suitable for high power and high efficiency power supplies. As LLC resonant power supplies are now being widely used and more and more people are asking questions about LLC transformer design. For this blog, I’ll discuss some frequently asked questions on this topic.
Transformer Saturation Issues
Q: My LLC transformer is designed to work in low magnetic induction intensity (Bm), but why does the temperature of the magnetic core become very high?
A: The LLC transformer works in LC resonance state. The LC resonance circuit features high Q, which is greater than 1 in your case. Thus, the voltage actually applied on the transformer is larger than the input voltage, which is a problem to be tackled when designing LLC transformers. Otherwise, the transformer will not work in magnetic induction intensity as you designed.
It is uncommon to have this saturation problem when the input voltage is high, as the switching frequency would also be high and the LC resonance circuit gain is low here. But, when the input voltage is low, the switching frequency will be low and the LC circuit gain is high, and in this case it is likely to have a saturation problem. So, when calculating the minimum turns of the inductance coil that you need, you must multiply your primary calculation by the gain factor. Furthermore, if leakage inductance is considered, it is better to multiply your result by the reciprocal of the coupling factor.
Choice of Wire
Q: Why does the temperature of the winding become very high during the aging test?
A: When an LLC transformer works in high frequency, the winding wire with an alternating magnetic field applied, experiences not only skin effect, but also proximity effect. The skin effect is the inherent behavior of the magnetic field going through the wire, while the proximity effect is induced by the magnetic field of the nearby conductor. Unlike flyback transformers, the primary winding of LLC transformers is on the same side, and the current in each of the winding turns flows in the same direction. The proximity effect becomes more significant as the number of winding layers increases. To solve this problem, you must use stranded wire with more wire cores.
Turns of Secondary Winding
Q: Why does the actual operating frequency deviate from the designed frequency?
A: This is a problem that involves many issues, and therefore it is difficult to explain in a few words. But, I find that designers like to determine first the turns of the primary winding, and then calculate the turns of the secondary winding according to the transformation ratio. The result obtained this way is often not an integer, so designers tend to simply round it up. This can create a large error in the transformation ratio, since the secondary winding usually has only a few turns. It is recommended that designers choose a proper integer for the secondary winding according to the calculation above, and then re-calculate the turns of the primary winding by the transformation ratio and then round up the result. Since the primary winding usually has more turns, the round-up error becomes relatively less significant.
No-load Voltage
Q: Why does my transformer have high light-load and no-load voltages?
A: This is also a complicated problem. One of the factors causing the problem might be the parasitic oscillation induced by the leakage inductance of the secondary winding and the parasitic capacitance among different winding layers or turns of the secondary winding. This is common in secondary windings with a large number of winding layers or turns. With light load, the parasitic oscillation is very strong, leading to an output voltage far beyond the designed value. To solve this problem, you can reduce the parasitic capacitance by isolating each secondary winding layer with rubber tapes, and you can also reduce the parasitic oscillation by using pile winding instead of parallel winding, for different winding directions.
I’d be happy to answer any more questions on LLC resonant transformers - please enter these as a comment to this blog.
You may find these other design resources for LLC resonant converters useful:
Power Seminar presentation and whitepaper:
Design Consideration of LLC Resonant Converter (You will find it in the Power Supplies section of the Online Seminars.) http://www.fairchildsemi.com/onlineseminars/index.html
Application Notes:
Analysis of MOSFET Failure Modes in LLC Resonant Converter http://www.fairchildsemi.com/an/AN/AN-9067.pdf#page=1
Half-bridge LLC Resonant Converter Design Using FSFR-series Fairchild Power Switch (FPSTM) http://www.fairchildsemi.com/an/AN/AN-4151.pdf#page=1
LLC Resonant Converter video:
Featuring zero voltage switching and low voltage stress, LLC resonant transformers are suitable for high power and high efficiency power supplies. As LLC resonant power supplies are now being widely used and more and more people are asking questions about LLC transformer design. For this blog, I’ll discuss some frequently asked questions on this topic.
Transformer Saturation Issues
Q: My LLC transformer is designed to work in low magnetic induction intensity (Bm), but why does the temperature of the magnetic core become very high?
A: The LLC transformer works in LC resonance state. The LC resonance circuit features high Q, which is greater than 1 in your case. Thus, the voltage actually applied on the transformer is larger than the input voltage, which is a problem to be tackled when designing LLC transformers. Otherwise, the transformer will not work in magnetic induction intensity as you designed.
It is uncommon to have this saturation problem when the input voltage is high, as the switching frequency would also be high and the LC resonance circuit gain is low here. But, when the input voltage is low, the switching frequency will be low and the LC circuit gain is high, and in this case it is likely to have a saturation problem. So, when calculating the minimum turns of the inductance coil that you need, you must multiply your primary calculation by the gain factor. Furthermore, if leakage inductance is considered, it is better to multiply your result by the reciprocal of the coupling factor.
Choice of Wire
Q: Why does the temperature of the winding become very high during the aging test?
A: When an LLC transformer works in high frequency, the winding wire with an alternating magnetic field applied, experiences not only skin effect, but also proximity effect. The skin effect is the inherent behavior of the magnetic field going through the wire, while the proximity effect is induced by the magnetic field of the nearby conductor. Unlike flyback transformers, the primary winding of LLC transformers is on the same side, and the current in each of the winding turns flows in the same direction. The proximity effect becomes more significant as the number of winding layers increases. To solve this problem, you must use stranded wire with more wire cores.
Turns of Secondary Winding
Q: Why does the actual operating frequency deviate from the designed frequency?
A: This is a problem that involves many issues, and therefore it is difficult to explain in a few words. But, I find that designers like to determine first the turns of the primary winding, and then calculate the turns of the secondary winding according to the transformation ratio. The result obtained this way is often not an integer, so designers tend to simply round it up. This can create a large error in the transformation ratio, since the secondary winding usually has only a few turns. It is recommended that designers choose a proper integer for the secondary winding according to the calculation above, and then re-calculate the turns of the primary winding by the transformation ratio and then round up the result. Since the primary winding usually has more turns, the round-up error becomes relatively less significant.
No-load Voltage
Q: Why does my transformer have high light-load and no-load voltages?
A: This is also a complicated problem. One of the factors causing the problem might be the parasitic oscillation induced by the leakage inductance of the secondary winding and the parasitic capacitance among different winding layers or turns of the secondary winding. This is common in secondary windings with a large number of winding layers or turns. With light load, the parasitic oscillation is very strong, leading to an output voltage far beyond the designed value. To solve this problem, you can reduce the parasitic capacitance by isolating each secondary winding layer with rubber tapes, and you can also reduce the parasitic oscillation by using pile winding instead of parallel winding, for different winding directions.
I’d be happy to answer any more questions on LLC resonant transformers - please enter these as a comment to this blog.
You may find these other design resources for LLC resonant converters useful:
Power Seminar presentation and whitepaper:
Design Consideration of LLC Resonant Converter (You will find it in the Power Supplies section of the Online Seminars.) http://www.fairchildsemi.com/onlineseminars/index.html
Application Notes:
Analysis of MOSFET Failure Modes in LLC Resonant Converter http://www.fairchildsemi.com/an/AN/AN-9067.pdf#page=1
Half-bridge LLC Resonant Converter Design Using FSFR-series Fairchild Power Switch (FPSTM) http://www.fairchildsemi.com/an/AN/AN-4151.pdf#page=1
LLC Resonant Converter video: