AN2625 Application note

Transcription

1 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 V AC 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 V, there is no safe voltage margin if the switcher is operated at 440 V RMS 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 V AC ) and low input voltage 90 V AC. It is intended to be used as an input stage with VIPerX2, VIPer53A-E based SMPS in cases when "ultra-wide" ranges (90 V RMS to 440 V RMS ) of input AC voltages are expected. January 2008 Rev 1 1/17

2 Contents AN2625 Contents 1 Circuit description Bill of material Evaluation results Waveforms at steady-state Waveform at start-up EMC evaluation EMC results - VIPer12A-E demo board load, output power 6 W EMC testing - resistive load, output power 10 W Conclusions Revision history /17

3 List of figures List of figures Figure 1. High input AC voltage limiting circuit Figure 2. Evaluation set-up Figure 3. Current and voltage waveforms at V IN = 440 V AC Figure 4. Current and voltage waveforms at V IN = 440 V AC with output voltage measurement Figure 5. Magnified view of Figure Figure 6. Maximum magnified view of Figure Figure 7. Voltages and currents at V IN = 90 V AC, Po = 6.4 W; no clamping - normal operation Figure 8. Voltages and currents at V IN = 90 V AC, Po = 9.44 W; no clamping - normal operation Figure 9. Voltage and current waveforms at start-up Figure 10. Peak detection, LINE, VIPer12A-E demo board as load Figure 11. Average detection, LINE, VIPer12A-E demo board as load Figure 12. Peak detection, NEUTRAL, VIPer12A-E demo board as load Figure 13. Average detection, NEUTRAL, VIPer12A-E demo board as load Figure 14. Peak detection, LINE, resistive load Figure 15. Average detection, LINE, resistive load Figure 16. Peak detection, NEUTRAL, resistive load Figure 17. Average detection, NEUTRAL, resistive load /17

4 Circuit description AN Circuit description The schematic of the circuit is shown in Figure 1. Figure 1. High input AC voltage limiting circuit L1 1 mh J1 1 Line 2 Line / Neutral V IN RMS = V L2 1 mh R1 30 Ω D1 4X1N V max 360 V DC A R2 620 kω Q1 R4 2N kω R3 620 kω C1 + C D V C2 220 nf /440 V AC 4.7 µf / 400 V D3 15 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 V RMS.(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 I DS 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

5 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 V DS (Figure 6). 1.1 Bill of material Table 1. 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 V AC 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 1.2 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 V IN RMS (V) V CLAMPED DC (V) P O (W) P IN (W) Efficiency (%) 20 kω kω EN55022, Class B , Rload = 8 kω 10 pass 1. Bulk capacitor, 10 µf / 400 V capacitor was used as C2 output capacitor. 5/17

6 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 V IN RMS (V) I IN RMS (ma) P IN (W) V C3MAX (clamp level) (V) Total P O (W) Efficiency (%) Above 260 V AC the limiting circuit is clamping the DC bus voltage of VIPer12A-E SMPS to 358 V max; At 80 V AC 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 V IN RMS (V) I IN RMS (ma) P IN (W) V C6MAX (clamp level) (V) Total P O (W) Efficiency (%) Above 260 V AC the limiting circuit is clamping the DC bus voltage of VIPer22A-E SMPS to 354 V max; At 80 V AC the VIPer22A-E SMPS is loosing regulation. At high line voltage 440 V AC the efficiency is decreased because of the combined energy losses in both the converter and clamping circuit. At low line voltage of 90 V AC, the efficiency is decreased because of the increased conduction loss in Q2 of the clamping circuit (Figure 7 and Figure 8). 6/17

7 Circuit description Figure 2. Evaluation set-up L1 1 mh J1 1 Line 2 Line / Neutral V IN RMS = V L2 R1 1 mh 30 Ω 220 nf/ 440 Vac D1 4X1N4007 C1 + C 630 V max 360 V A DC R2 620 kω D2 5.6 V R5 7.5 kω Q1 2N3904 D3 15V R3 620 kω C2 4.7 µf / 400 V Q2 + STP4NK50Z B C µ F / 400 V 4 L3 45 mh L4 30 mh C3 10 µf/ 400 V + C5 22 µf/ 400 V TEST 1 (Table 3) AN2103: VIPer12A-E Demo board; 6 W, 12 V / 0.5 A output; Efficency: 75% specifications. TEST 2 (Table 4) AN1736: VIPer22A-E Demo board; 10 W, 5 V / 1.0 A, 12 V / 0.42 A output; Efficency: 75% specifications. +12 V / 0.5 A +5 V / 1.0 A +12 V / 0.42 A R4 22 Ω R6 5 Ω R7 30 Ω AI /17

8 Circuit description AN Waveforms at steady-state All measurements are referenced to the schematic shown on Figure 1. Figure 3. Current and voltage waveforms at V IN = 440 V AC Ch1. - the voltage after the rectifier bridge. Figure 4. Current and voltage waveforms at V IN = 440 V AC 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

9 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

10 Circuit description AN2625 Figure 7. Voltages and currents at V IN = 90 V AC, Po = 6.4 W; no clamping - normal operation Figure 8. Voltages and currents at V IN = 90 V AC, 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

11 Circuit description Waveform at start-up Test conditions: V IN 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 R DS(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

12 EMC evaluation AN EMC evaluation The clamping circuit was tested for conducted EMI compliance, according to EN 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 V AC 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 V AC with the clamped voltage set to 300 V DC. 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 6 W Figure 10. Peak detection, LINE, VIPer12A-E demo board as load Figure 11. Average detection, LINE, VIPer12A-E demo board as load 12/17

13 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

14 EMC evaluation AN EMC testing - resistive load, output power 10 W Figure 14. Peak detection, LINE, resistive load Figure 15. Average detection, LINE, resistive load 14/17

15 EMC evaluation Figure 16. Peak detection, NEUTRAL, resistive load Figure 17. Average detection, NEUTRAL, resistive load 15/17

16 Conclusions AN 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. Document revision history Date Revision Changes 18-Jan Initial release 16/17

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