Output Filters and Power Failure Warning CircuitsIntroductionWhy are output filters needed?What’s out in the IndustryOutput Low-pass filterTwo-Stage filterResonant filterCommon-mode noise filter (1)Common-mode noise filter (2)Design ExampleDesign Example cont’dSimulation of Common-mode noise filterSimulation ResultsPower Failure Warning CircuitsLine Failure and Brownout ConditionsSimple Power Failure Warning CircuitDynamic Power Failure Warning CircuitIndependent Power Failure Warning CircuitFast Power Failure Warning CircuitConclusionOutput Filters and Power Failure Warning CircuitsBy: Carmen LeeEE 136, Section 01San Jose State UniversityFall 2003Introduction •The following chapters from Keith Billings’ Switchmode Power Supply Handbook will be discussed: Chapter 20 on output filters and Chapter 21 on power failure warning circuits. •The advantages and disadvantages of each type of circuit will be discussed along with the power supply problems which they solve.Why are output filters needed? •Output filters are needed in switchmode supplies to reduce ripple and conducted-mode noise. •The types of output filters to be discussed are:–Output low-pass filter –Two-stage filter –Resonant filter –Common-mode noise filter •Output filters can be found at SMPS and Maxim•Power failure warning circuits can be found in microprocessors made by Philips, National Semiconductor, and Linear TechnologyWhat’s out in the IndustryOutput Low-pass filter•A simple low-pass filter consists of an inductor and a capacitor as shown below.• This type of filter is effective in filtering low and medium frequency range noise, but it is not effective in filtering noise at high frequency due to ESR and ESL of large capacitors (recall from Real Components lecture). –The solution to reduce these parasitic effects would be to use more expensive capacitors.Two-Stage filter•A two stage filter is an improvement of the output low-pass filter and is a cost effective way in filtering out low, medium, and high frequency noise. •Second stage L2 and C2 are small so that it filters high-frequency.Resonant filter•A resonant filter is just a modification of the two-stage filter to bring the self-resonant frequency near the switching frequency to provide better supply performance.•Uses low ESR capacitorsCommon-mode noise filter (1)•For the common-mode noise filter, the inductor of the filter needs to be split to balance the filter. •More capacitors are needed “between each output line and ground to provide a return path for the residual common-mode noise current” (Billings, 1.156). •Therefore, this filter uses more componentsCommon-mode noise filter (2)Design Example•Design example of a forward converter given by Billings.•Specifications–Output Power = 100 W–Output Voltage = 5 V–Output Currrent = 20 A–Operating Frequency = 30 kHz–Minimum load = 20%–Assume output current ripple = 30% and–Ripple voltage = 500mVDesign Example cont’d• where Vs if the secondary voltage.• VL=Vs-Vout=16.66-5=11.66V where VL is the inductor voltage.• where L is the required inductance, t is the “on” time, and i is the current change during the “on” time.•The capacitor is determined by the ripple current and the ripple voltage specification. By assuming the ripple voltage to be 500mV, the following equation can be used to determine the capacitance.•This capacitor will be sufficient if only the ripple voltage were the requirement, however, if the change in current needs to be considered as in most cases, a larger capacitor will be needed. The calculations are shown below.)(2121222omCLVVCLIEEFVVLICom7092536205.192222FVtICoon1205.0106HitVLL4.1961066.11VtTVVonouts66.161033.335Simulation of Common-mode noise filterR1C1C2L1+ VVMout+ VVM1C3L2PULSE1S1D1E2C4L325k250 u75u.1 u.1u470 u10R2R3R4•The following shows a Buck converter with a common-mode noise filter attached to it.Simulation ResultsVM1.V [V] t [s] 16-2246810120 0.110m 20m 30m 40m 50m 60m 70m 80m 90mVMout.V [V] t [s] 10-124680 0.110m 20m 30m 40m 50m 60m 70m 80m 90mPower Failure Warning Circuits•Power failure circuits provide extra time needed to shut down the system.•Types of power failure warning circuits to be discussed are:–Simple–Dynamic–Independent–FastLine Failure and Brownout Conditions•Two important terms to know before discussing about power failure warning circuits are:•Line failure (instantaneous failure to zero or near zero voltage)•Brownout (a fall in line voltage to below minimum value then recovery to normal)•Brownout failure (a fall in line voltage until the circuit fails)Simple Power Failure Warning Circuit•Simple power failure warning circuit are only suitable for total line failure conditions •AC line input into bridge rectifier to provide a pulsating current through OC1. OC1 maintains pulsating current to drive transistor while C2 is pumped low to keep Q2 on.Dynamic Power Failure Warning Circuit•Dynamic power failure warning circuits respond to all conditions.•It uses a comparator amplifier that compares the voltage through C1 and C2 to a reference voltage. The output of the amplifier drives the optical coupler to generate a warning signal.Independent Power Failure Warning Circuit•Independent power failure warning circuits are more precise than the previous circuits mentioned and provides better temperature coefficient.Fast Power Failure Warning Circuit•Fast power failure warning circuits provide the earliest warning of brownout or failure.•It detects the rate of change of the input voltage.FIG. 1.21.6 (b) “Optimum speed” power failure warning circuit for direct operation from ac line inputs.Conclusion•Output filters–Best solution is the two stage filter because of its cost effectiveness•Power failure warning circuits–Best solution is the fast power warning failure circuit because it responds the quickest to brownout or