cd00171116

AN2625
Application note
High AC input voltage limiting circuit
Introduction
The requirements on the switched mode power supply applications regarding the input AC
voltage range are constantly increasing: for example, the ultra wide input voltage range of
90 to 440 VAC is now commonly required in many applications (electricity meters, lighting,
factory automation). This inevitably leads to an increased working voltage levels and thus
limited safe operation of the power supply components. This application note is describing
one possible approach that can be used for safe operation of primary switchers with high
input voltages.
The circuit is aiming to limit (clamp) input AC voltages higher than 260 V to levels safe for
the operation of ST off-line SMPS primary switchers (e.g. VIPer12A-E, VIPer22A-E,
VIPer53A-E). It is presumed that the input AC voltage can reach 440 V maximum (line-toline voltage of a 3phase power net 230 / 400 V +10% tolerance). As the maximum
breakdown voltage of the embedded VIPer MOSFET is between 620 - 730 V, there is no
safe voltage margin if the switcher is operated at 440 VRMS input voltage.
The circuit is based on using a MOSFET (N-channel 500 V, 2.7 Ω Power MOSFET,
STP4NK50Z) working as a low frequency (100 Hz) switch and a comparator setting the
clamped high voltage level. The comparator is built by using a combination of NPN transistor
and a zener diode (Figure 1). The circuit is tested by supplying two different ST switchers demo board VIPer12A-E (AN 2103, Ref.2) and demo board VIPer22A-E (AN 1736, Ref.3),
connected to the input bulk capacitors. Accordingly, a minimum changes were introduced to
the original configuration of the demo boards by the connection of the clamping circuit. The
circuit was also tested under simple resistive loads.
The proposed circuit strives to achieve efficiency higher than 50% in the power range of 5 to
10 W at high input voltage (up to 440 VAC) and low input voltage 90 VAC. It is intended to be
used as an input stage with VIPerX2, VIPer53A-E based SMPS in cases when "ultra-wide"
ranges (90 VRMS to 440 VRMS) of input AC voltages are expected.
January 2008
Rev 1
1/17
www.st.com
Contents
AN2625
Contents
1
2
Circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1
Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2
Evaluation results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1
Waveforms at steady-state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2.2
Waveform at start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
EMC evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1
EMC results - VIPer12A-E demo board load, output power 6 W . . . . . . . 12
2.2
EMC testing - resistive load, output power 10 W . . . . . . . . . . . . . . . . . . . 14
3
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2/17
AN2625
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
High input AC voltage limiting circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Evaluation set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Current and voltage waveforms at VIN = 440 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Current and voltage waveforms at VIN = 440 VAC with output voltage measurement . . . . . 8
Magnified view of Figure 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Maximum magnified view of Figure 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Voltages and currents at VIN = 90 VAC, Po = 6.4 W; no clamping - normal operation . . . . 10
Voltages and currents at VIN = 90 VAC, Po = 9.44 W; no clamping - normal operation . . . 10
Voltage and current waveforms at start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Peak detection, LINE, VIPer12A-E demo board as load . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Average detection, LINE, VIPer12A-E demo board as load . . . . . . . . . . . . . . . . . . . . . . . . 12
Peak detection, NEUTRAL, VIPer12A-E demo board as load . . . . . . . . . . . . . . . . . . . . . . 13
Average detection, NEUTRAL, VIPer12A-E demo board as load . . . . . . . . . . . . . . . . . . . 13
Peak detection, LINE, resistive load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Average detection, LINE, resistive load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Peak detection, NEUTRAL, resistive load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Average detection, NEUTRAL, resistive load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3/17
Circuit description
1
AN2625
Circuit description
The schematic of the circuit is shown in Figure 1.
Figure 1.
High input AC voltage limiting circuit
630 V max
L1
L2
R1
1 mH
1 mH
30 Ω
J1
220 nF /440 VAC
2 Line / Neutral
VIN RMS = 90 - 440 V
–
+
C
D2 5.6 V
R4
7. 5 kΩ
360 VDC
R3
620 kΩ
R2
620 kΩ
D1
4X1N4007
C1
1 Line
A
Q1
2N3904
C2
D3
15 V
+
4.7 µF /
400 V
Q2
STP4NK50Z B
AI14514
In steady state the MOSFET Q2 works as an 100 Hz operated switch interrupting the charge
of C2 (the bulk capacitor at the input of the SMPS). Before the voltage at point C reaches
the threshold level of 6.3 V the MOSFET Q2 is ON - Q1 is OFF sinking minimum Icbo
current and the zener D3 breaks down to 15 V assuring stable ON state of Q2. This is also
the normal operating condition for the MOSFET at input AC voltages ≤ 260 VRMS.(The AC
level of the input voltage at which the 6.3 V level is reached, is chosen to be 360 V for this
evaluation. Accordingly, 360 V is the level at which the input AC voltage is limited). The
threshold level is set up by the zener voltage of D2 and B-E junction voltage of Q1. The
negative temperature coefficient of the B-E junction partially cancels the breakdown zener
diode's positive temperature coefficient (Vz > 5 V), thus achieving a temperature
independence of the threshold level.
As it can be seen from Figure 3 and Figure 4 (paragraph 1.2. Waveforms at steady state),
the IDS current flows only for a fraction of the whole conduction ON time interval of Q2 - the
current flows only when the voltage across C2 becomes less than the rectified input AC
voltage allowing for the rectifying diodes of D1 bridge to be forward biased. Accordingly, C2
starts charging during that time.
If there were not a switching circuitry (R2, R4, D2, Q1), the voltage across C2 would follow
the input AC rectified waveform until the rectifying diodes of the D1 bridge were reverse
biased. But the input AC voltage reaches the 360 V threshold level and the MOSFET Q2 is
turned OFF - at voltage ≥ 6.3 V at point C, Q1 turns ON diverting and sinking the whole
current from the zener D3, the Vgs of Q1 drops to zero and Q2 is switched OFF. The
rectifying diodes of D1 bridge are still forward biased but they do not conduct current as the
MOSFET Q2 is OFF. Accordingly, the voltage across C2 gets limited as there is no charging
current flowing.
The rectifying diodes of D1 bridge are forward biased up to the moment when the rectified
input AC voltage starts to fall below the voltage across C2. Hence, a time interval follows
when both the rectifying diodes and the MOSFET are OFF. At the moment when the
rectified input AC voltage falls below the threshold level of 360 V (i.e. 6.3 V at point C), the
4/17
AN2625
Circuit description
MOSFET is turned ON again but current does not flow as the rectifying diodes are still
reverse biased. C2 discharges at a rate determined by the output power level. Eventually,
the voltage across C2 decreases enough to forward bias the bridge D1 rectifying diodes and
thus allowing short charging current pulses to flow and replenish the energy loss of C2 to
the limited value.
It can be seen from Figure 5 and Figure 6, the switching power dissipation of Q2 is very
small - during every switching period the MOSFET is ON only for 450 µs. No temperature
rise of Q2 is observed. Hence, the overall power loss of the circuit is determined by the
passive components rather then by the switch Q2.
The current thru the switch Q2 is sharply interrupted when the rectifying diodes are forward
biased. As they are 50 Hz rectifying diodes, this current interruption causes the ringing
observed on VDS (Figure 6).
1.1
Bill of material
Table 1.
1.2
BOM for 360 V limited level
Item
Qty
Ref.
Description
Manufacturer
1
2
L1, L2
1mH / 0.2 A choke
General purpose component
2
1
C1
220 nF / 440 VAC
General purpose component
3
4
D1
1N4007
General purpose component
4
1
Q1
2N3904
General purpose component
5
1
D2
5.6 V / 0.5 W Zener Diode
General purpose component
6
1
D3
15 V / 0.5 W Zener Diode
General purpose component
7
1
Q2
STP4NK50Z
STMicroelectronics
8
1
C2
4.7 µF / 400 V
General purpose component
9
1
R4
7.5 kΩ/ 0.25 W, 5%
General purpose component
10
2
R2, R3
620 kΩ / 0.25 W, 5%
General purpose component
Evaluation results
The circuit uses the component values shown on Figure 1 and Table 1. The clamped output
voltage was set to 360 V. Under resistive loading, the following measurements were made,
Table 2:
Table 2.
Measurements with resistive load(1)
Resistive load
VIN RMS (V)
VCLAMPED DC (V)
PO (W)
PIN (W)
Efficiency (%)
20 kΩ
440
350
6
7.3
82
13 kΩ
440
348
9.15
10.7
85
EN55022, Class B
260
285, Rload = 8 kΩ
10
—
pass
1. Bulk capacitor, 10 µF / 400 V capacitor was used as C2 output capacitor.
5/17
Circuit description
AN2625
Next, the circuit was loaded with non-linear loads, Figure 2. The following measurements
were made, when the load of the clamping circuit is the VIPer12A-E SMPS, output power of
6.4 W (Table 3):
Table 3.
Measurements with VIPer12A-E reference design as load
VIN RMS (V) IIN RMS (mA)
PIN (W)
VC3MAX (clamp level) (V)
Total PO (W)
Efficiency (%)
440
144
9.66
358
6.4
66
380
126
9.32
358
6.4
68
320
124
9.19
358
6.4
69
260
72
8.72
358
6.4
73
Above 260 VAC the limiting circuit is clamping the DC bus voltage of VIPer12A-E SMPS to 358 V
max;
200
69
8.31
274
6.4
77
160
80
8.17
220
6.4
77
100
130
8.75
138
6.4
73
90
145
9.1
120
6.4
70
At 80 VAC the VIPer12A-E SMPS is loosing regulation.
When the load of the clamping circuit is the VIPer22A-E SMPS, output power 10 W, the
evaluation results are shown in Table 4:
Table 4.
Measurements with VIPer22A-E reference design as load
VIN RMS (V) IIN RMS (mA)
PIN (W)
VC6MAX (clamp level) (V) Total PO (W)
Efficiency (%)
440
195
15.6
354
9.6
61
380
192
15.45
354
9.6
62
320
182
15.14
354
9.6
63
260
98
14.22
354
9.6
67
Above 260 VAC the limiting circuit is clamping the DC bus voltage of VIPer22A-E SMPS to 354 V
max;
200
104
13.25
268
9.6
72
160
118
12.72
216
9.6
75
100
168
13.95
132
9.6
68
90
211
14.4
110
9.6
66
At 80 VAC the VIPer22A-E SMPS is loosing regulation.
At high line voltage 440 VAC the efficiency is decreased because of the combined energy
losses in both the converter and clamping circuit. At low line voltage of 90 VAC, the efficiency
is decreased because of the increased conduction loss in Q2 of the clamping circuit
(Figure 7 and Figure 8).
6/17
1 Line
30 Ω
R1
220 nF/ 440 Vac
1 mH
1 mH
2 Line / Neutral
VIN RMS = 90 - 440 V
J1
L2
L1
D1
4X1N4007
C1
–
+
C
R5
7.5 kΩ
Q1
2N3904
D2 5.6 V
R2
620 kΩ
630 V max
STP4NK50Z B
C2
4.7 µF /
D3 400 V
15V
Q2
10 µ F /
400 V
C4 +
+
360 VDC
R3
620 kΩ
A
1
1
2
L4
4 30 mH 3
2
L3
4 45 mH 3
+
+
22 µF/
400 V
C5
10 µF/
400 V
C3
AN1736:
VIPer22A-E Demo board;
10 W, 5 V / 1.0 A,
12 V / 0.42 A output;
Efficency:
75% specifications.
TEST 2 (Table 4)
AN2103:
VIPer12A-E Demo board;
6 W, 12 V / 0.5 A output;
Efficency:
75% specifications.
+12 V / 0.42 A
+5 V / 1.0 A
+12 V / 0.5 A
AI14515
R7
30 Ω
R6
5Ω
R4
22 Ω
Figure 2.
TEST 1 (Table 3)
AN2625
Circuit description
Evaluation set-up
7/17
Circuit description
1.2.1
AN2625
Waveforms at steady-state
All measurements are referenced to the schematic shown on Figure 1.
Figure 3.
Current and voltage waveforms at VIN = 440 VAC
Ch1. - the voltage after the rectifier bridge.
Figure 4.
Current and voltage waveforms at VIN = 440 VAC with output voltage
measurement
Ch1 - the voltage across C2.
The other channels are measuring: - Ch.2 - gate-source voltage of Q2; Ch3 - drain-source
voltage of Q2; Ch4. - the drain-source current peaks thru Q2.
8/17
AN2625
Circuit description
Figure 5.
Magnified view of Figure 4
Ch3. - drain-source voltage of Q2.
Figure 6.
Maximum magnified view of Figure 4
Ch.3.- drain-source voltage of Q2, note the ringing.
The other channels are measuring: - Ch.2 - gate-source voltage of Q2; Ch.1 - VC2 voltage;
Ch4. - the drain-source current peaks thru Q2.
9/17
Circuit description
AN2625
Figure 7.
Voltages and currents at VIN = 90 VAC, Po = 6.4 W; no clamping - normal
operation
Figure 8.
Voltages and currents at VIN = 90 VAC, Po = 9.44 W; no clamping - normal
operation
The channels are as follows: - Ch.1 - the voltage across C2; Ch.2 - gate-source voltage of
Q2; Ch3. - Q2 drain-source voltage; Ch.4 - Q2 drain-source current peaks.
10/17
AN2625
1.2.2
Circuit description
Waveform at start-up
Test conditions: VIN RMS = 440 V; VIPer12A-E SMPS at full load (max. power 6.4 W).
Figure 9.
Voltage and current waveforms at start-up
Figure 9 shows a start-up sequence. In all cases the inrush current reaches about 6 A peak
- the only inrush limiting device is R1 (30 Ω, 2 W) and the RDS(ON) of Q2 (< 2.7 Ω).
The channels are as follows: - Ch.1 - the voltage across C2; Ch.2 - gate-source voltage of
Q2; Ch3. - Q2 drain-source voltage; Ch.4 - Q2 drain-source current peaks.
11/17
EMC evaluation
2
AN2625
EMC evaluation
The clamping circuit was tested for conducted EMI compliance, according to EN 55022
Class B using Peak and Average detection. It should be noted that the only measure for EMI
suppression implemented in the circuit, are the simple chokes L1 and L2 - 1 mH / 0.2 A
(Figure 1). The capacitor C1 220 nF / 440 VAC is a simple snubber element across the
rectifying diodes of D1 bridge. The EMI filters in the demo boards were preserved in their
original place as shown in Figure 2.
The circuit was tested at maximum 260 VAC with the clamped voltage set to 300 VDC.
VIPer12A-E demo board, output power of 6 W, was the load of the clamping circuit.
For output power of 10 W, the clamping circuit was tested under resistive loading (Table 1).
2.1
EMC results - VIPer12A-E demo board load, output power
6W
Figure 10. Peak detection, LINE, VIPer12A-E demo board as load
Figure 11. Average detection, LINE, VIPer12A-E demo board as load
12/17
AN2625
EMC evaluation
Figure 12. Peak detection, NEUTRAL, VIPer12A-E demo board as load
Figure 13. Average detection, NEUTRAL, VIPer12A-E demo board as load
13/17
EMC evaluation
2.2
EMC testing - resistive load, output power 10 W
Figure 14. Peak detection, LINE, resistive load
Figure 15. Average detection, LINE, resistive load
14/17
AN2625
AN2625
EMC evaluation
Figure 16. Peak detection, NEUTRAL, resistive load
Figure 17. Average detection, NEUTRAL, resistive load
15/17
Conclusions
3
AN2625
Conclusions
As it can be seen from Table 2 and Table 3, the circuit shows total efficiency of 60% to 68%
in the power range of 5 W to 10 W. Provided the SMPS efficiency is 80%, the overall
efficiency can be expected to rise to more than 70%.
The evaluation results show that the proposed low components count circuit can
successfully limit high input AC voltages in the power range of up to 10 W and accordingly,
buffered with this circuit the primary switchers can easily work above the levels they have
been originally designed for.
4
Revision history
Table 5.
16/17
Document revision history
Date
Revision
18-Jan-2008
1
Changes
Initial release
AN2625
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