Isolated Power Supply world-wide AC input

Part 5

Clamping Diode Network

Figure 72

Current in clamping diode network. Two TVS diodes (150V+150V) connected in series.
1.7Apk under high line voltage

Figure 73

Board came in a slight modificated condition. The diode clamping network made of 150V + 150V TVS in series. 400mApk under low line voltage

Experiments - Changing Clamping Network

Figure 74

I vs. U curve of a 150V TVS diode on a curve tracer.

Figure 75

350V TVS Diode I vs. U curve

Figure 76

350V+150V TVS Diode in series connection I vs. U curve

Figure 77

Using now only one 150V TVS Clamping Diode.

Figure 78

With only one clamping diode (TVS150) hard clamping. Drain voltage reduced compared to 150V+150V solution. More current in the clamping diode.

Figure 79

With only one clamping diode (TVS150) more clamping action. Drain voltage reduced compared to 150v+150V solution. More current in the clamping diode.

Figure 80

With the TVS150 only, Drain peak voltage reduced to 540Vpk, reduce voltage stress in the MOSFET and would improve EMC. Much power in clamping diode.

Gate voltage rise up slow for a slow MOSFET Switch-ON, there is no need to switch on fast, because Drain voltage and Drain current is low and therefore no Switch-ON loss.

Gate voltage rise-down for a fast MOSFET Switch-OFF, there is a need for a fast switch off, to keep switching loss in a reasonable limit.

Comparison Table

	@260VAC rms	@260VAC rms	@85VAC rms	@85VAC rms
	Drain pk voltage	Clamping Diode pk Current	Drain pk voltage	Clamping Diode pk Current
TVS150V+TVS150V in series	680Vpk (Figure 73)	0.34Apk (Figure 73)	425Vpk (Figure 72)	1.7Apk (Figure 72)
TVS150V only	540Vpk (Figure 80)	1.2Apk (Figure 79)	310Vpk (Figure 77+78)	2.2Apk (Figure 78)

Figure 81

The harder clamping reduce the MOSFET voltage stress, transfer power to the Clamping diode. Propably will improve EMC.

Measuring Peak Power Clamping Diode

Figure 82

Clamping Diode TVS150V, voltage and current waveform. Diode peak power 396Watt pk. Peak Power to much for repetitive clamping function, require PCB heat-sink under repetitive 396Wpk.

Figure 83

Clamping Diode TVS150V, voltage and current waveform. Diode peak power 201 Watt pk.

Comparison Table, renewed

	@260VAC rms	@260VAC rms	@260VAC rms	@85VAC rms	@85VAC rms	@85VAC rms
	Drain pk voltage	Clamping Diode pk current	Clamping Diode pk power	Drain pk voltage	Clamping Diode pk current	Clamping Diode pk power
TVS150V+TVS150V in series	680Vpk (Figure 73)	0.34Apk (Figure 73)	128Wpk tp=0.05ms (no Figure)	425Vpk (Figure 72)	1.7Apk (Figure 72)	528Wpk tp=0.1ms (no Figure)
TVS150V only	540Vpk (Figure 80)	1.2Apk (Figure 79)	201Wpk tp=0.2ms (Figure 83)	310Vpk (Figure 77+78)	2.2Apk (Figure 78)	396Wpk tp=0.2ms (Figure 82)

Figure 84

Two series diode sharing power. With higher clamping voltages, pulsewidth become smaller.
Choosing the right TVS value a game of power distribution and EMC.

Output Electrolytic Capacitor

Figure 85

Current waveform of the output electrolytic capacitor, 2200µF/50V. Rms current creates capacitor heating. Stay well below maximum limit to prevent capacitor from heating-up - will increase capacitor lifetime.

Here 1ms/Div. * 10 Divisions = 10ms, that is exactly 50Hz*2 =100Hz exactly one cycle ==> therefore rms and mean value correct for the setted oscilloscope timebase.

Mean must be zero, same amount of In and Out capacitor currents.

Figure 86

AC coupled output voltage, 1.359 Vpkpk. 100Hz ripple plus switching frequency ripple.

Figure 87

Switching Current Ripple into the Electrolytic Output Capacitor 37.5mV*2A/Div. = 7.5A pk. AC-Switching-Ripple voltage approx. 180mVpk

Resistance = 0.18Vpk / 7.5Apk = 0.024 Ohm
Equivalent Series Resistor (ESR) = 0.024 Ohm @ f=100-500kHz
Capacitor Datasheet value = 0.019 Ohm for 25°C, 100 kHz

Measurement fits well to the Datasheet value, consider also the real measurement has PCB wires, solder junctions and a inductive current loop wire added to the measurement - both values fitting excellent to each other.

Figure 88
measuring current with shortest current probe wire and 1:10 voltage probe.

Output Voltage Spectrum

Figure 89

FFT Spectrum of Capacitor current and voltage, Span 10Hz to 100kHz.

What we see here?

small 50Hz (mainly transfered by air)
very large 100Hz (AC-rectified Ripple)
200Hz-800Hz (AC-rectified harmonics, AC line harmonics)
50kHz (switching frequencies starting here)
100kHz (second switching harmonic)

Figure 90

Span 10kHz to 30MHz, we see 50kHz-2MHz (switching frequencies)

Blue Capacitor voltage curve shows a significant pole, capacitor impedance rise up at 2MHz. Caused by capacitor+Layout.
Output clean in higher frequency areas. Used oscilloscope a 200MHz/12Bit resolution oscilloscope, 1Meg Inputs.

Figure 91

Power heating up the capacitor. Difference between ohmic thermal power and LC-power doesn´t matter here. Any DC-offset in the current probe and ADC-input offset voltage will create an additional measurement error.

Increasing to 260VAC rms

Figure 92

With 260VAC rms, voltage and current waveforms looking quite different compared to 85VAC rms. Capacitors has less peak power, but more often.

Figure 93

With 260VAC rms, voltage and current spectrum looking different compared to 85VAC rms. That means EMC tests should be done under different AC line voltages.

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