Switcher Efficiency & Snubber Design 1 Agenda • • • • • • • SMPS Basics Control Methods Losses Example: Buck Example: Boost BJTs vs. MOSFETs Snubber Design 2 SMPS Basics 3 The goal of a converter is to deliver power It ’s getting hot in here! Pin Pout The conversion mechanism generates heat... 4 Heat means that the energy transfer is not perfect η=Pout/Pin is called the efficiency Ploss = Pin − Pout = * 5 Pout η ⎛1 ⎞ − Pout = Pout ⋅ ⎜⎜ − 1⎟⎟ ⎝η ⎠ A 50% efficiency means Ploss = Pout e.g. Pout = 100 W ÎPloss = 100 W Pin = 150 W, Pout = 100 W Îη = 66% Two different options exist to build a converter: R The linear approach: Î efficiency is poor Î good noise performance 0 to ∞ Î acceptable when (Vout-Vin) is small Î can only decrease the input level The switching approach: Î efficiency is high ON or OFF Î noise performance is poor Î works with large (Vout-Vin) Î increase/decrease/invert the input level 6 The linear approach: Help! I am dissipating 500 W!! (0) Vout 10 A 10 A 50 V 100 V η=50% 7 The switching approach: Hey! I feel much cooler now... Vout 6A 10 A 100 V 50 V Vin η=83% Vout Vout 0 8 Controlling the power flow 1 Voutavg = ⋅ Ts Ts ∫ 0 Vout (t ) ⋅ dt = ton ⋅Vin = D ⋅Vin Ts ton toff Ts ton (Duty-cycle) D= Ts Pulse Width Modulation (PWM) control 9 Control Methods 10 Regulation, keeping an output signal constant by… • • • • • • adjusting the duty-cycle via the PWM block regulating the inductor peak current regulating the inductor average current adjusting the switching frequency off time adjustment … Current-mode control… Two most popular Voltage-mode control… methods! 11 The voltage-mode method I > Imax? Î reset The error level sets the duty-cycle 12 The duty-cycle factory… Verror plot1 verr, ramp in volts 6.50 PWM modulator 4.50 2.50 497m -1.50 7.00 + 100% 5.00 plot2 out in volts - 2 1 3.00 0% 1.00 3 -1.00 10.0u 30.0u 50.0u time in seconds 70.0u 90.0u A ramp is compared to a DC level, the error voltage 13 The current-mode method The error level sets the current setpoint 14 The peak follows the error voltage 3.00 3.50 2.00 2.50 1.00 vfb in volts plot1 idrain in amperes 1 idrain 2 vfb 4 idrain ge Error volta 2 1 1.50 0 500m -1.00 -500m 1.31m S Q 3 vfb 1.65m 1.99m time in seconds 2.33m 2.67m MOSFET Q 3.50 3.00 2.50 2.00 1.50 idrain in amperes Plot2 vfb in volts 3 4 R 1.00 500m 0 -500m -1.00 Inductor peak current 2.50m 15 2.57m 2.65m time in seconds 2.72m 2.80m Losses 16 Losses Pout = Pin − PSW − PCon − PIC PSW: Switching Losses PCon: Conduction Losses PIC: Power consumed by the chip 17 IC Losses PIC = Vin I q Vin: IC input voltage Iq: Quiescent current (read from the data sheet) 18 Switching Losses Y-Axis • Losses which occur when the power switch is turned on or off. • During this transition the voltage and current on the FET are both high. • Different for Buck and Boost configurations. ton Current(DS) Voltage(DS) Time 19 Switching Losses (Buck) PSW 1 = Vin I out (ton + toff ) FSW 2 Vin: Input Voltage Iout: Average inductor current ton: Turn on time of high side switch toff: Turn off time of high side switch FSW: Switching frequency 20 Switching Losses (Boost) PSW I out 1 = Vout (ton + toff ) FSW 2 1− D Vout: Output Voltage D: Duty Cycle Iout: Average inductor current ton: Turn on time of high side switch toff: Turn off time of high side switch FSW: Switching frequency 21 Reducing Switching Losses • Increase gate drive strength – Increases cost (die area) – Increases EM emissions • Decrease frequency – Requires a larger value inductor • Use a smaller FET – Increases conduction losses 22 Conduction Losses • Losses which occur when current flows through a resistive path (I2*R), such as a FET, or a diode (V*I). • Major contributors include: – Power Switch Rds,on – Freewheeling Diode – Inductor DCR • Different for synchronous and non-synchronous mode designs. 23 Conduction Losses (Non-Synchronous) PCon = I L Rds ,on D + I LVdiode (1 − D) + I L RDCR 2 IL: RMS current through the inductor Rds,on: On resistance of the power switch D: Duty cycle Vdiode: Forward voltage of the diode RDCR: Winding resistance of the inductor 24 2 Conduction Losses (Synchronous) PCon = I L Rds ,on1 D + I L Rds ,on 2 (1 − D) + I L RDCR 2 2 IL: RMS current through the inductor Rds,on1: On resistance of the high side switch D: Duty cycle Rds,on2: On resistance of the low side switch RDCR: Winding resistance of the inductor 25 2 Example: Buck 26 Example: NCV8851 Evaluation Board • • • • • • • Synchronous buck converter Vin = 13.2 V Vout = 5 V Iout,max = 4 A FSW = 170 kHz Inductor: Wurth Electronics 7447709150 15 uH Power switch (both): ON Semiconductor NTD5407N 27 Example: NCV8851 Evaluation Board • • • • D = Vin / Vout = 5 V / 13.2 V = 0.379 ton = 5 ns (empirical) toff = 7 ns (empirical) Rds,on1 = Rds,on2 = 50 mΩ (including self heating temperature effects) • RDCR = 26 mΩ • Iq = 15 mA • IL = Sqrt(Iout2 + (0.609 A)2 / 3) 28 Example: NCV8851 Evaluation Board • • PIC = 13.2 V * 15 mA = 0.198 W PSW = (1/2) * 13.2 V * Iout * (5 ns + 7 ns) * 170 kHz = 0.01346 * Iout W PCON = (Iout2 + 0.12378 A2) * (50 mΩ) * (0.379) + (Iout2 + 0.12378 A2) * (50 mΩ) * (0.621) + (Iout2 + 0.12378 A2) * 26 mΩ = (0.076 * Iout2 + 0.009407) W Sources of Power Dissipation 1.4 1.2 Power (W) • 1 Piq Psw Pcon 0.8 0.6 0.4 0.2 0 0 1 2 Iout (A) 29 3 4 Example: NCV8851 Results Efficiency 8851 Efficiency 100.00% 90.00% 80.00% 70.00% 60.00% 50.00% 40.00% 30.00% 20.00% 10.00% Actual Calculated 0.00% 0.000 1.000 2.000 Iout (A) 30 3.000 4.000 Example: Boost 31 Example: NCV8871 Sample Application • • • • • • • • Non-synchronous boost converter Vin = 13.2 V Vout = 18 V Iout,max = 9 A FSW = 170 kHz Inductor: Vishay IHLP6767GZER330M11 33 uH Power Switch: ON Semiconductor NTD5803 x 2 Diode: ON Semiconductor MBRB1645T4G 32 Example: NCV8871 Sample Application • • • • • • • D = 1 – (Vin / Vout) = 0.267 ton = 30 ns toff = 20 ns Rds,on = (12 mΩ) / 2 = 6 mΩ (including temperature effects) RDCR = 37 mΩ Iq = 10 mA IL = Sqrt ((Iout / (1 – D)) ^ 2 + (0.3137)^2 / 3) 33 Example: NCV8871 Sample Application • PIC = 13.2 V * 10 mA = 0.132 W PSW = (1/2) * 18 V * Iout / (1-0.267) * (20 ns + 30 ns) * 170 kHz = 0.10436 * Iout W PCON = (Iout2 + 0.0328 A2) * (12 mΩ) * (0.267) + Sqrt(Iout2 + 0.0328 A2) * (0.5V) * (0.733) + (Iout2 + 0.0328 A2) * 37 mΩ = (0.0402 * Iout2 + Sqrt(Iout2 + 0.0328)*0.3665 + 0.00131869 W Power Dissipation 6 5 Power(W) • • 4 Pic Psw Pcon 3 2 1 0 0 3 6 Iout(A) 34 9 Example: NCV8871 Results Efficency 100.00% Efficiency 80.00% 60.00% Calculated 40.00% 20.00% 0.00% 0 3 6 Iout(A) 35 9 BJTs vs. MOSFETs 36 Switches and converters… The bipolar transistor is often used: 1. In high voltage - high current applications 2. In low-cost converters Ic Pcond = Vcesat ⋅ Icavg = Vce When saturated… The bipolar transistor 37 Switches and converters… 1. 2. 3. The bipolar transistor switching losses: Depend on temperature (storage time, current tail) Watch-out for hot spots! Often needs proportional drive (shallow saturation) Vce Ic losses Psw = ton The bipolar transistor 38 toff Vce ⋅ Ic ⋅ (ton+toff ) 3 ⋅ Tsw If ton = toff… Switches and converters… 1. 2. 3. The MOSFET transistor is the most popular: Ease of drive (capacitive input) Avalanche rugged BVdss of 600 V for SMPS, 500 V for PFCs… d Often used in freewheel function d Pcond =IdRMS²⋅Rds(ON) Rds(ON) = g body diode s g s The MOSFET transistor 39 To enhance a MOSFET, bring it charge Plateau level Miller effect d Crss Coss g VT Ciss s How many coulombs to turn on the MOSFET: Q = i x t… 40 Snubber Design 41 When is it needed? • Parasitic inductances and capacitances from the power devices form a RLC filter that resonates • Excessive ringing can cause damage to the devices 42 Snubber Design • Measure the frequency of the ringing (fc) at maximum input voltage – Use a low capacitance probe • Find out either the L or C of the circuit – L is dominated by the top power switch – C is dominated by the body diode of the bottom power switch or the capacitance of the freewheeling diode • Calculate the characteristic impedance of the circuit – If L is known: Z = 2πfc L – If C is known: Z = 1 / ( 2 πfc C ) 43 Snubber Design • • • • Choose RSNUB = Z Choose CSNUB = 1 / (2 πf R) Power dissipation in RSNUB is CV2fs Put RSNUB and CSNUB in series across the device causing ringing. • Test in circuit. RSNUB can be fine turned further to reduce ringing if it is found to be insufficient 44 For More Information • View the extensive portfolio of power management products from ON Semiconductor at www.onsemi.com • View reference designs, design notes, and other material supporting automotive applications at www.onsemi.com/automotive 45