AN052 Reduction of No-load Power Consumption The Future of Analog IC Technology Application Note for Reduction of No-load Power Consumption Prepared by Hommy Ding Oct 09, 2011 AN052 Rev. 1.0 12/29/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 1 AN052 Reduction of No-load Power Consumption The Future of Analog IC Technology ABSTRACT This note presents a method of reducing no-load power consumption for flyback converters. Under normal operation, power loss of a flyback converter includes conduction loss, switching loss and control circuit loss. In no-load condition, the current in the circuit is very small which makes the conduction loss almost negligible. Switching loss and control circuit loss are major sources of power loss, and must be minimized to reduce no-load power consumption. AN052 Rev. 1.0 12/29/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 2 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION INDEX ABSTRACT ............................................................................................................................... 2 NO-LOAD POWER LOSS ANALYSIS...................................................................................... 4 Xcap ........................................................................................................................................ 4 Input Capacitor ....................................................................................................................... 4 RCD Snubber .......................................................................................................................... 5 Switching Components.......................................................................................................... 6 (1) MOSFET ............................................................................................................................ 6 (2) Diode.................................................................................................................................. 8 5. Transformer ............................................................................................................................ 9 6. Control Circuit ...................................................................................................................... 11 (1) IC Controller ..................................................................................................................... 11 (2) Feedback Circuit .............................................................................................................. 11 1. 2. 3. 4. EXAMPLE ............................................................................................................................... 13 APPLICATION SUGGESTIONS ............................................................................................. 15 Discharging Resistors ................................................................................................................ 15 Electrical Capacitor .................................................................................................................... 15 Switch Component ..................................................................................................................... 15 Transformer ................................................................................................................................ 15 Control Circuit ............................................................................................................................ 15 AN052 Rev. 1.0 12/29/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 3 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION NO-LOAD POWER LOSS ANALYSIS The analysis presented in this note is based on a flyback converter with an MPS current mode controller. For a flyback converter working under no-load condition, power losses can be divided into six parts: (1) Xcap discharge loss, (2) Input capacitor loss, (3) RCD snubber loss, (4) Loss of switching components, (5) Transformer power loss, (6) Loss of control circuit. 1. Xcap An Xcap is a kind of safety capacitor connected between L and N. It acts as a filter on the differential mode interference of the power supply. If the capacitance exceeds 0.1μF, when input is disconnected, the circuit automatically discharges the capacitor to avoid any potential electrical shock, the discharging time must not exceed 1s for pluggable power supplies. The relevant time constant is the product of the effective capacitance and the discharging resistance in the circuit. However, because determining the effective capacitance and resistance values precisely is difficult and the Xcap is usually the dominant capacitance, then we can estimate time constant as a function of the Xcap and the discharging resistors. So that the discharging time constantτ of a RC network is then: τ = R x ⋅ Cx (1) Where Cx is the Xcap, Rx is the discharging resistance. As the time constant τ can not exceed 1s, the discharging resistors must be smaller than 1/Cx. The discharging resistors continuously dissipate power throughout operation. The power dissipated by the discharging resistors Pdischarge can be calculated as: Pdisch arg e VAC 2 = Rx (2) Where VAC is the rms value of the AC input voltage. The power loss through discharging resistor contributes significantly to the no-load power loss especially in the high-input condition. To decrease the no-load power consumption, increase the discharging resistance, through in some instances the Xcap must decrease in order to increase the discharging resistance, which may deteriorate EMI performance. As a compromise, choose the appropriate Xcap and discharging resistors according to each application. 2. Input Capacitor Power loss of electrical capacitor induced by the leakage current IR can not be ignored when the capacitor voltage is very high. To decrease the no-load power consumption, lower the leakage current of the input capacitor as much as possible. The leakage current IR can be calculated as: IR = K ⋅ Cin ⋅ Vin (3) Where K is the coefficient of leakage current, Cin is the capacitance, and Vin is the DC input voltage. We can obtain the loss induced by the input capacitor PCapacitor as: PCapacitor = K ⋅ Cin ⋅ Vin2 (4) For a flyback converter, Cin is defined by the power of the converter, and Vin is defined by the rms value of the AC input voltage. Therefore, the coefficient K dominates the loss induced by the capacitor. It is correlative with material purity of capacitor and use condition. At typical temperature, K is 0.01 for a AN052 Rev. 1.0 12/29/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 4 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION general specific product, and it is 0.0001 for a premium product. The loss induced by the capacitor can be several to tens of mW. Choose a capacitor with a low leakage current to minimize the standby power loss. 3. RCD Snubber In operation, the energy stored in the leakage inductance can not transfer to the output side of a flyback converter. This energy may result in a high voltage spike across the MOSFET and the rectifier diode, which can cause severe EMI noise and device failure. The RCD snubber shown in Figure 1 suppresses the voltage spike to protect the component. + Vclamp Rsn Csn Dsn Lleakage Rsn Csn Lleakage D Figure 1: RCD Snubber on Primary and Secondary Side The RCD snubber dissipates the energy of the leakage inductance and limits the voltage spikes. Accurate analysis of the RCD snubber power loss is affected by the leakage inductance, snubber diode and parasitic capacitance, but can be roughly estimated by assuming the energy stored in the leakage inductance is completely dissipated by the RCD snubber circuit in steady state. The energy stored in the leakage inductance can be expressed as: Pleakage = 1 ⋅ Lleakage ⋅ Ip r i _ peak 2 ⋅ f 2 (5) Where Lleakage is the leakage inductance, Ipri_peak is the primary peak current, and f is the switching frequency. When the converter works in no-load condition, the current sense voltage threshold Vpeak can be obtained from the datasheet of the IC controller. So the primary peak current Ipri_peak is determined by the sense resistor Rsense. The peak current Ipri_peak under no-load condition is given as: Ip r i _ peak = AN052 Rev. 1.0 12/29/2014 Vpeak Rsense www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. (6) 5 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION Meanwhile, the converter enters burst mode when working in no-load condition. An equivalent switching frequency fs can be used to substitute the switching frequency f and be calculated by (5). fs = Nsw Nsw ⋅ t sw + t burst (7) As shown in Figure 2, Nsw is the number of switchings in one burst time, tsw is the switching period and tburst is the burst time. Vgs NSW 0 tsw t tburst Figure 2: Vgs in Burst mode By substituting (4) (5) into (3), we can obtain the energy stored in the leakage inductance as: Pleakage = V Nsw 1 ⋅ Lleakage ⋅ ( peak )2 ⋅ 2 Rsense Nsw ⋅ t sw + t burst (8) 4. Switching Components Generally, switching components include the MOSFET and diode in a flyback converter. (1) MOSFET Power loss of MOSFET can be divided into conduction loss, switching loss and gate driving loss. As mentioned above, the primary peak current in no-load condition can be calculated with the current sense voltage threshold Vpeak to derive the conduction loss of MOSFET. Usually, the gate drive stage is integrated in the controller so the gate driving loss of MOSFET can be also included in the loss of the control circuit. Figure 3 shows the flyback transformer magnetizing current at no-load condition; generally it is in DCM mode. Ip 0 ton toff tsw t Figure 3: Magnetizing Current in Transformer AN052 Rev. 1.0 12/29/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 6 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION The primary side MOSFET on time ton and secondary side diode on time toff can be calculated as: Lm ⋅ Ipri _ peak t on = t off = Vin Lm ⋅ Ipri _ peak N ⋅ (Vout + VF ) (9) (10) Where Lm is the transformer magnetizing inductance, N is the transformer turn ratio (primary side to secondary side), Vout is the output voltage, and VF is the forward voltage drop of secondary diode. The primary side current can be calculated as: Ipri (t) = Vin ⋅ t,0 ≤ t ≤ t on Lm (11) From (7) (9), the primary side rms and average current can be obtained as: Ipri _ rms = 1 t SW Ipri _ avg = ton ∫I pri (t)2 dt (12) (t)dt (13) 0 1 t SW t on ∫I pri 0 The conduction loss of MOSFET PMOSFET_conduction in no-load condition is given as: PMOSFET _ conduction = Ipri _ rms 2 ⋅ Rds(on) (14) Where Rds(on) is the on state resistance of MOSFET. The converter works in DCM mode in no-load condition. That means that the switch turns on in zerocurrent condition. So the turn on power loss of MOSFET is dominated by the equivalent primary-side parasitic capacitance Coss which includes the MOSFET junction capacitance, transformer parasitic capacitance, diode junction capacitance etc. In no load condition, the converter usually operates in deep DCM mode, and the MOSFET drain-source voltage is Vds. The power loss during MOSFET turnon is: PMOSFET _ Coss = AN052 Rev. 1.0 12/29/2014 1 ⋅ Coss ⋅ Vds 2 ⋅ fs 2 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. (15) 7 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION Figure 4 shows the voltage and current waveforms of MOSFET during turn-off. VDS Vds IDS Ip Td(off) Tf Figure 4: Voltage and Current Waveforms of MOSFET during Turning off The switching off loss of MOSFET PMOSFET_switching is: PMOSFET _ switching = 1 ⋅ Vds ⋅ Ip ⋅ (t d(off ) + t f ) ⋅ fs 2 (16) We can find turn-off delay time td(off) and fall time tf in the datasheet of MOSFET with a given gate resistance. fs is the equivalent switching frequency given in (7). (2) Diode For low-output-voltage application, use a Schottky diode to reduce the conduction loss and avoid potential diode reverse-recovery problem. The diode works in zero-current condition. The diode switchoff loss can be ignored and the snubber capacitance Cdiode dominates the switch-on loss. So the power loss during diode turn-on is: 2 Pdiode _ sw int ch = 1 ⎛V ⎞ ⋅ Cdiode ⋅ ⎜ in + Vout ⎟ ⋅ fs 2 ⎝ N ⎠ (17) The conduction loss of diode is induced by the forward voltage drop VF of diode. VF always changes with the forward current. For simplicity, use a constant value to calculate the power loss. In no-load condition, VF is the voltage of diode under the secondary peak current Isec_peak. With the primary peak current Ipri_peak, the secondary peak current Isec_peak can be calculated as: Isec _ peak = N ⋅ Ipri _ peak AN052 Rev. 1.0 12/29/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. (18) 8 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION Figure 5 shows the diode current waveform. V GS 0 t on t sw Diode Current(A) I secondary 0 t on t on+toff Figure 5: Current Waveform of Diode The average value of secondary current can be written as follow: Isec _ avg 1 ⋅ I sec _ peak ⋅t off =2 ts (19) The conduction loss of diode in no-load condition can be obtained as: PDiode _ conduction = VF ⋅ Isec_ avg (20) 5. Transformer Transformer power loss can be divided into copper loss Pcopper and core loss Pcore. The copper loss is caused by the winding resistance. The resistance contains DC impedance and AC impedance. So the copper loss also contains DC loss and AC loss. The DC loss can be obtained as: Pcopper _ DC = Ipri _ rms 2 ⋅ Rpri _ winding + Isec_ rms 2 ⋅ Rsec_ winding (21) Where Rpri_winding is the resistance of primary winding, Rsec_winding is the resistance of secondary winding, which are given as: Rp r i _ winding = ρcopper ⋅ Rsec_ winding = ρcopper ⋅ AN052 Rev. 1.0 12/29/2014 Np r i ⋅ Lbobbin Npri _ strand ⋅ Spri _ copper Nsec ⋅ Lbobbin Nsec_ sec tion ⋅ Ssec_ copper www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. (22) (23) 9 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION Where z ρcopper is the conductivity of copper, z Npri is the number of primary turns, z Nsec is the number of secondary turns, z Lbobbin is the mean length of the turn, z Npri_strand is the strands of primary winding wire, z Nsec_strand is the strands of secondary winding wire, z Spri_copper is the cross section area of single primary winding wire, z Ssec_copper is the cross section area of single secondary winding wire. The analysis of AC loss is difficult to calculate because of the difficulty in calculating the AC impedance and AC current accurately. Based on the AC transformer winding resistance calculation model of Dowell, the AC power loss can be calculated as: Pcopper _ AC = Ip r i _ ACrms 2 ⋅ αR p r i _ winding +Isec_ ACrms 2 ⋅ αRsec_ winding (24) Where α is the empirical factor to estimate the AC resistance due to the calculation model. It is about 1.5 to 2 in this note. The rms AC current value of primary and secondary side can be can be estimated by: Ip r i _ ACrms = Ipri _ rms 2 − Ip r i _ avg2 (25) Isec_ ACrms = Isec_ rms 2 − Isec_ avg2 (26) The core loss can be calculated with an empirical formula as below: PCore = Cm ⋅ (fs )x ⋅ (Bmax )y ⋅ (Ct0 − Ct1 ⋅ TCore + Ct 2 ⋅ TCore 2 ) ⋅ Ve (27) Where Cm, x, y, Ct0, Ct1, and Ct2 are coefficients related to the material of core, Bmax is the maximum magnetic flux, Tcore is the temperature of core. AN052 Rev. 1.0 12/29/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 10 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION 6. Control Circuit (1) IC Controller The power loss of IC controller contains internal IC consumption and the loss induced by the start up circuit. For internal IC consumption, the power loss can be calculated as: PIC = VCC ⋅ ICC (28) Where Vcc is the supply voltage of the IC controller, Icc is the operation current in no-load condition. To decrease the no-load power consumption, select the lowest-possible voltage. This voltage must be higher than the lowest operating voltage of the IC controller. Select the voltage VCC using the turn-ratio between the secondary and auxiliary winding for a given output voltage. The IC controller needs a circuit to start up. Some ICs have an HV pin, some have startup resistors. Both the circuits will consume the power. For the ICs with an HV pin, we can find the leakage current Ileakage from HV pin in the datasheet. The loss induced by the leakage current can be estimated as: PLeakage _ current = Vin ⋅ ILeakage (29) Where Vin is the DC input voltage. Minimize the leakage current on the HV pin to decrease this power loss. However, it is mainly determined by the chip process. For the IC with startup resistors, estimate the loss induced by the startup resistors as: Pstartup ( V − VCC ) = in 2 (30) Rstartup To decrease this power loss, use large startup resistors. However, large startup resistors slow the startup speed and may even result in startup failure. (2) Feedback Circuit For a flyback converter with isolated output, typically adopt an optocoupler and three-terminal programmable shunt regulator like the TL431 to achieve output voltage feedback. For example, the HFC0300 feedback circuit shown in Figure 6 consumes some power for normal operation. To minimize the no-load power consumption, minimize the power loss of the feedback circuit. For an isolated flyback converter, the output voltage usually powers the optocoupler and the regulator. If the converter has multiple outputs, choose the lower voltage as the supply voltage. Alternatively, choose a regulator with a lower operating current and an optocoupler with a high current transfer ratio (CTR). Vout VCC R1 R2 Vcomp Ccomp V ref Rcomp PGND SGND Figure 6: Feedback Circuit AN052 Rev. 1.0 12/29/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 11 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION For the HFC0300, Vcomp is about 3.1V when the converter is working in burst mode condition. The primary and secondary optocoupler current can be estimated as: Isec _ photocoupler = Ip r i _ photocoupler = Vcomp Rcomp Isec _ photocoupler CTR (31) (32) Where CTR is the current transfer ratio of the optocoupler. The power loss of feedback circuit contains two parts as shown below: Pfeedback = VCC ⋅ Isec _ photocoupler + Vout ⋅ Ip r i _ photocoupler (33) Using a regulator with a low operating current and high CTR optocoupler, the primary and secondary side current of the feedback circuit drops and reduces the power loss. If the voltage supply for the feedback circuit is very high (typically range 12V- 24V), we can save 10mW-20mW power loss by choosing a regulator with a lower operating current and an optocoupler with a higher CTR. AN052 Rev. 1.0 12/29/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 12 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION EXAMPLE In order to show the validity of no-load consumption analysis, a flyback converter controlled by the HFC0300 was built and tested. The AC input is 90 Vrms to 264Vrms; the outputs are 5V/3A and 24V/1.5A respectively. The circuit of the converter is shown in Figure 7. L N 1 1 1 F1 2 RT1 5.1 CX1 0.22uF 1M R1A 1 2 1M R2A LX1 32mH 3 4 2 BD1 3 3 R5 1 R4 1 Q1 R3 1 P1065ATF 1 4 1 2 R7 10 C1 100uF 2 C2 1 R6 20K R8 JR3 JR2 3.48K 3.48K RF2 R9 2.2nF C4 D1 1 1 2 HV FR107 N/C D2 DRV VCC FR107 CS 10K GND FSET U3 COMP 200K 1 2 3 33pF 4 2 1K 1 3 5 6 R20 0 R10 1 8 7 6 C7 5 1 EER28 12 11 8 7 10 9 R27 4 2 3 51 R12 R11 0 10K C19 C3 0.1uF CY4 4.7nF T1 C20 470pF 47uF 10nF 3 1 D3 1nF R16 2.2 D4 C8 MBRF10150CT 3 1 SP1060 2 1 2 2 1 1 C12 1000uF L1 3.3uH 2 C15 2 1 L2 3.3uH 220uF 1 C14 2 1000uF 2 C16 2 3 220uF 1 2 2K R15 R17 10K 1 2.2nF C10 C11 1000uF U1 C9 10nF PC817A 1 2 1K R14 1 R13 20K U2 TL431 2 1uF C17 1uF C18 R23 274K 12.4K R24 1 2 1 2 CN2 CN3 Figure 7: Circuit of HFC0300 Controlled Flyback Converter AN052 Rev. 1.0 12/29/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 13 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION The measured no-load power consumption was 91.5mW at 264Vrms AC input. To decrease the standby power loss, some measure is given as: 1) Increase the discharging resistors to 4MΩ. 2) Optimize the transformer design. 3) Choose 5V output voltage as the supply voltage of feedback circuit and use a regulator of 1.24V reference. The measured no-load power consumption decreased to 51.7mW, and the waveform of burst mode at no-load condition is as shown in Figure 8. CH1:Vds CH1:Vds CH2:Vcc CH2:Vcc CH3:Vcomp CH3:Vcomp Figure 8: Waveform of Burst Mode at No-load We can obtain the equivalent switch frequency fs from the waveform. As per the analysis of no-load power consumption previously discussed, we can calculate the no-load power loss of each part and summarized them in Table 1. Table 1: No-Load Power Loss Breakdown Input Voltage 264VAC fs 107Hz No-load power loss breakdown Discharging Input capacitor 17.42mW resistor RCD snubber MOSFET 0.15mW Diode Transformer 1.76mW IC(HFC0300) Feedback circuit 15.34mw 7.27mW 3.46mW 0.12mW 1.07mW Thus, we can find the loss through the discharging resistor and IC are the major part of total no-load power loss. However, with the increased equivalent frequency, the power loss of MOSFET, diode and transformer increase significantly and dominates. AN052 Rev. 1.0 12/29/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 14 AN052 – REDUCTION OF NO-LOAD POWER CONSUMPTION APPLICATION SUGGESTIONS Follow the suggestions below to decrease the no-load power consumption by decreasing the loss on each component. Discharging Resistors Small discharging resistors will cause higher power loss. Choose a suitable Xcap to optimize the noload loss and EMI. Electrical Capacitor Choose an appropriate capacitor with relatively low leakage current, balanced against increased cost. Switch Component z Choose a MOSFET with low Rds(on), high switching speed, and low output capacitance. z Use an application-appropriate gate drive resistor for the MOSFET to balance efficiency against EMI. z Use a Schottky diode with a low forward voltage drop Transformer z Choose an appropriate winding size and use multiple strands of wire z Use a transformer with a sandwich winding structure to decrease the leakage inductance z Choose a low loss core material Control Circuit z Optimize the IC losses by decreasing the loss on the startup circuit and the operation current in noload condition. z Use a regulator with a low operating current and an optocoupler with a high-CTR z Design an appropriate feedback circuit to decrease the equivalent frequency as much as possible. NOTICE: The information in this document is subject to change without notice. Users should warrant and guarantee that third party Intellectual Property rights are not infringed upon when integrating MPS products into any application. MPS will not assume any legal responsibility for any said applications. AN052 Rev. 1.0 12/29/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 15