advertisement

High-Frequency Link Inverter Based on Multiple-Carrier PWM Philip T. Krein, Xin Geng, Robert Balog University of Illinois March 2002 Outline • The PWM cycloconverter. • Dual-carrier PWM to build waveforms for HF links. • Properties of dual-carrier signals for gate drives and other purposes. • Experimental results. • Conclusions. The PWM Cycloconverter • High-frequency (HF) link inverters can be constructed as: – A cascade of a high-frequency dc-dc converter and inverter. – A “square-wave cycloconverter,” in which a high-frequency square-wave inverter provides the input to a cycloconverter. High Frequency Inverter Inverter Load (a) DC/DC converter type High Frequency Inverter CycloConverter (b) Cycloconverter type Load The PWM Cycloconverter • The dc-dc converter alternative has multiple power conversion stages. • The cycloconverter would seem to have complicated operation, since it is treated as a nonlinear phase control problem. • The complexity has been one factor limiting use. • Now consider a conventional PWM inverter. A two-level inverter has a single input (Vin) and can produce an output of Vin. • There should be some way to work from an input of Vin and generate exactly the same output waveform. The PWM Cycloconverter • This is the PWM cycloconverter: use as input a simple square wave input at high frequency, then control the switches to produce an output that is exactly a conventional two-level PWM waveform. • The PWM cycloconverter is not a new concept. • What is new is that conventional PWM can be extended to cycloconverter operation with a multiple-carrier PWM process. Dual-Carrier PWM • Consider the use of two separate PWM waveforms, modulated with a desired low-frequency waveform m(t). • This could be separate rising and falling ramps, triangles with phase shifts, or the like. • Call these Carrier 1 and Carrier 2. • Now modulate both Carrier 1 and Carrier 2 with the signal m(t), to give PWM 1 and PWM 2. • The sum PWM 1 + PWM 2 has low-frequency content 2m(t). • The difference PWM 1 – PWM 2 has no low-frequency content. Dual-Carrier PWM • Is the result trivial? Not if we use time multiplexing to make sure the final waveform retains switching behavior. • Several choices of combinations are available. Decommutator C1(t) Comparator gate drive sequence P1 +/carrier modulating function o 0/180 Phase Shifter 1 C2(t) 0/180o Phase Shifter 2 0/180o Phase Shifter 3 HF link voltage input M1(t) M2(t) Comparator P2 Sw itching Control cycloconverter voltage output Dual-Carrier PWM • There are systematic ways to develop specific desirable properties in the final output waveform. • Example: – Create two carriers from a single ramp just by blanking every other pulse. – Modulate both with m(t), then subtract. – The result is a three-level high-frequency link signal. • Another example: – Split a triangle into separate rising and falling ramps. – Modulate, respectively, with m(t) and –m(t). – This yields a two-level signal “PWM” signal with no lowfrequency content. Dual-Carrier PWM C lo ck (squ a rew av e ) T r iang le F irs t ca rr ie r S econd ca rr ie r C om p a re f irs t to s ign a lm ( t) C om p a re second to s ign a l m - ( t) Dual-Carrier PWM • The PWM sum has 50% duty, but retains the information. C om p a re f irs t to s ign a lm ( t) C om p a re seco n d to s ign a l m - ( t) U se o n e com p a ra to r to se t , th e o th e r to re se t . T h is is w t o -ca r r ie r PW M ( t ) . T h e o r ig in a l c lo ck Dual-Carrier PWM • • • • What about the desired two-level PWM output? Use the square wave clock as the input to a cycloconverter. Use the sum waveform PWM(t) as the gate control. This “convolution” process of clock and PWM(t) recovers the desired two-level PWM output. U se o n e com p a ra to r to se t , th e o th e r to re se t . T h is is w t o -c a r r ie r PW M ( t ) . T h e o r ig in a l c lo ck C lo ck co n v o lv ed w ith PW M ( t ) . T h is is co n v en t io n a l PW M ,w ith e a ch tu rn -o f f adv an c ed re la t iv e to th e c lo ck . Dual-Carrier PWM • The PWM waveform is this example is also always phaseadvanced with respect to the original square wave. • List the combinations for two-carrier PWM. TABLE 1. COMBINATION CONDITONS (WITH DECOMMUTATOR) AND RESULTING TWO-CARRIER PWM SEQUENCES Carrier Type Phase Shifter 1 Phase Shifter 2 Phase Shifter 3 Combining Method Gate Drive Signal Type Output PWM Equivalent 1 Triangle 0 0 180º Add 2-Level Ramp PWM at double fswitch 2 Triangle 0 180º 0 Add 2-Level Ramp PWM at double fswitch 3 Triangle 0 0 0 Subtract 3-Level Triangle PWM 4 Triangle 180º 0 180º Add 2-Level Ramp PWM at double fswitch 5 Triangle 180º 180º 0 Add 2-Level Ramp PWM at double fswitch 6 Triangle 180º 0 0 Subtract 3-Level Triangle PWM 7 Ramp 0 0 180º Add 2-Level Triangle PWM 8 Ramp 0 180º 0 Add 2-Level Triangle PWM 9 Ramp 0 0 0 Subtract 3-Level Ramp PWM at double fswitch Dual-Carrier PWM • Triangle-based carrier sets with output delay and advance, respectively. comparing C1(t) and M1(t) P1(t) comparing C2(t) and M2(t) P2(t) gate drive sequence HF link voltage output voltage (a) (b) Dual-Carrier PWM • Three-level PWM examples for HF links. comparing C1(t) and M1(t) P1(t) comparing C2(t) and M2(t) P2(t) gate drive sequence HF link voltage output voltage (c) (d) Properties in the Two-Carrier Case • We can select among several properties: – By using carriers that alternately control turn-on and turnoff, gate waveforms with 50% duty can be generated. – The combined signal can have pure advance or delay. – The resulting PWM output can be generated with an effectively doubled switching frequency. Properties in the Two-Carrier Case 1. The two-carrier process allows conventional PWM modulators, combined with some simple logic, to generate waveforms for PWM cycloconverters. 2. With use of both advanced and delayed gating waveforms, natural commutation can be supported, in a manner equivalent to conventional sine wave SCR cycloconverters. 3. The cases that yield 50% duty ratio gating signals are especially valuable for transformer gate drives. Experimental Results • The two-carrier technique has been used to build a simple “naturally commutated PWM cycloconverter.” • This represents a high-frequency link inverter that makes use of conventional PWM to provide control – with no intermediate dc-dc converter. • SCRs are used – only the leading edge of the combined two-carrier signal is needed for the gate drives. (IGBTs could have been used instead.) Experimental Results • NCC square-wave cycloconverter (three-phase). Inverter S1 S3 Cycloconverter HF-TR Q1 Q3 LC Filter Load E S2 S4 Q2 Q4 Experimental Results • Test circuit, single-phase output. Cycloconverter Q1 HF-TR Q5 Q8 Gate Pulse for Q1 25 60mH Square waveform Q3 current sensor Load Gate Drive Gate Drive Q6 Q4 Q2 Q7 Gate Drive Gate Drive SGN(i) + - LM311 Ramp m(t) + -1 -m(t) PWM1 LM311 + - PWM2 LM311 Mono- Qd stable 1/2 74LS221 Qa Gate Drive Gate Pulse for Q3 Gate Pulse for Q4 Gate Pulse for Q5 Gate Pulse Qd Mono- Qa stable 1/2 74LS221 15 s Gate Pulse for Q2 Gate Drive 15 s Gate Pulse Gate Pulse for Q6 Gate Drive Gate Pulse for Q7 Gate Drive Gate Pulse for Q8 Experimental Results • The modulation process. • Only the leading edges are needed. Experimental Results • The crossover behavior from delayed gating to advanced gating. Experimental Results • Devices here switch at 3750 Hz. • Output is twolevel PWM at 7500 Hz. Conclusion • A multiple-carrier method can be used to generate PWM gating signals with a variety of properties. • Two-carrier signals chosen to cancel the baseband signal m(t) support high-frequency link inverters. • These inverters, PWM cycloconverters, eliminate a power stage but produce conventional PWM output waveforms. • The control complexity is similar to familiar PWM, but with multiple signal paths. Conclusion • The multiple-signal output can be tailored for useful properties, such as gate drives with 50% duty ratio under all modulating signals, and outputs that provide an effective doubling of the switching frequency.