potcore
Electrical
- Feb 26, 2007
- 28
Dear Engineers
Our bench power supplies having been going missing from work (people are allowed to “work from home”) and I have been asked to make some. I have tried out a SMPS power supply with TL497, but have encountered problems in my investigations….
(My scope is a cheap $200 (new) type, and my readings are perhaps a little rough, though indicative.)
I have just designed and tested a flyback SMPS using TL497 controller IC to give V(out) = 8V with V(in) = 5V
The Flyback schematic is as follows………………………………
When the secondary load was ~2K resistive, the circuit waveforms and V(out) appear to be in order.
However, when the secondary load was 24R, the waveforms are unexpected and V(out) drops to ~2V.
I would be grateful if engineers could possibly shed some light….
Primary & Secondary were wound on an RM14 potcore (no air gap)
L(primary) was ~ 18mH (about 50 turns 056mm enam. copper wire)
L(secondary) was ~ 23mH (about 64 turns 056mm enam. copper wire)
Primary and secondary coil resistance ~ 1R
V(out) was intended to be 8V
V(in) was 5V.
The power switch and diode used were those internal to the TL497.
The “negative” rails of primary and secondary circuits were tied together since the voltage feedback from the secondary was direct (no opto-isolator).
Flyback waveforms with Secondary load ~2Kohms resistance…………….
Flyback waveforms with Secondary load ~24R ohms resistance…………….
Since the rate of current rise in the primary when the switch is closed should be 278A/second (= V(in) / L), and since with either load, the switch was obviously never ON for more than 15uS (see switch waveform) , -then I would expect the peak primary current to never be higher than ~4mA.
-However, the primary current sense resistor voltage goes up to 200mV , meaning the peak primary current was 200mA.
This “overcurrent” indicates saturation, However, if saturation occurred, then I would expect the primary current to keep on rising, and not “clamp” at 200mA.
When the load is 24R, The primary current sense resistor voltage shows that the primary current does indeed STOP rising at 200mA and then REMAINS at that value for a while.
I cannot understand why the primary current stops rising at 200mA, (when surely the core is saturated and it should go higher), and then just goes along at that level for a while.
I would be very grateful if any thoughts could be offered.
Our bench power supplies having been going missing from work (people are allowed to “work from home”) and I have been asked to make some. I have tried out a SMPS power supply with TL497, but have encountered problems in my investigations….
(My scope is a cheap $200 (new) type, and my readings are perhaps a little rough, though indicative.)
I have just designed and tested a flyback SMPS using TL497 controller IC to give V(out) = 8V with V(in) = 5V
The Flyback schematic is as follows………………………………
When the secondary load was ~2K resistive, the circuit waveforms and V(out) appear to be in order.
However, when the secondary load was 24R, the waveforms are unexpected and V(out) drops to ~2V.
I would be grateful if engineers could possibly shed some light….
Primary & Secondary were wound on an RM14 potcore (no air gap)
L(primary) was ~ 18mH (about 50 turns 056mm enam. copper wire)
L(secondary) was ~ 23mH (about 64 turns 056mm enam. copper wire)
Primary and secondary coil resistance ~ 1R
V(out) was intended to be 8V
V(in) was 5V.
The power switch and diode used were those internal to the TL497.
The “negative” rails of primary and secondary circuits were tied together since the voltage feedback from the secondary was direct (no opto-isolator).
Flyback waveforms with Secondary load ~2Kohms resistance…………….
Flyback waveforms with Secondary load ~24R ohms resistance…………….
Since the rate of current rise in the primary when the switch is closed should be 278A/second (= V(in) / L), and since with either load, the switch was obviously never ON for more than 15uS (see switch waveform) , -then I would expect the peak primary current to never be higher than ~4mA.
-However, the primary current sense resistor voltage goes up to 200mV , meaning the peak primary current was 200mA.
This “overcurrent” indicates saturation, However, if saturation occurred, then I would expect the primary current to keep on rising, and not “clamp” at 200mA.
When the load is 24R, The primary current sense resistor voltage shows that the primary current does indeed STOP rising at 200mA and then REMAINS at that value for a while.
I cannot understand why the primary current stops rising at 200mA, (when surely the core is saturated and it should go higher), and then just goes along at that level for a while.
I would be very grateful if any thoughts could be offered.
