This paper analyzes the losses of a stepping inductor converter in the presence of load transients. Stepping inductor converter is a buck VRM topology which only employs a single phase. Stepping inductor converter is very effective in dealing with fast transient and at the same time it can give better steady state efficiency. Occurrences of transient load current changes are measured in a PC and data are used for the comparison to evaluate the real world performance of the stepping inductor converter and the multiphase interleaved buck converter.published_or_final_versio

Law, YY

Liu, JCP

Lu, DDC

Pong, MH

Poon, NK

HKU Scholars Hub

Title Loss analysis of a single phase fast transient VRM converterAuthor(s) Law, YY; Lu, DDC; Liu, JCP; Poon, NK; Pong, MHCitationConference Proceedings - Ipemc 2004: 4Th International PowerElectronics And Motion Control Conference, 2004, v. 3, p. 1161-1165Issued Date 2004URL http://hdl.handle.net/10722/45828Rights Creative Commons: Attribution 3.0 Hong Kong LicenseLoss AnaIysis of a Single Phase Fast Transient VRM Converter Y .  Y .  Law, Dylan D.C. Lu, Joe C.P. Liu, N.K. Poon, and M.H. Pong Power Electronics Laboratory Department of Electrical and Electronic Engineering University of Hong Kong Pokfulam Road, Hong Kong, China mhp@eee.hku.hk , Abstract- This paper analyzes the loss& of a stepping inductor converler in the presence of load fransienb. Stepping inductor converter is a buck VRM fopobgy which only employs n single plmse. Stepping inductor converter is very effective in dealing wifk fast transiertf mid at tlre same tune i f  can give heuer steady sfnte efficiency. Occurrences of lrarrsient load currenl chmiges are measured in a PC arid a h  are used for the comparison to evaluate the real world performance of fhe sfepping ifductor converter a i d  the muhiplmse interleaved buck convirter. Keywords - Buck converter, jiuf fmnsieru, stepping inductor, VRM I. INTRODUCTION In order to tackle the issues of fast transient several voltage regulator module (VRM) topologies have been proposed. The multiphase interleaved topology [I] is currently the most popular. However. it needs several “phases” with more power devices as well as control chainels. A stepping inductor topology has been proposed recently [2] which einploys a single phase only which has simpler configuration aid has been shown to be very effective. It has been proved that stepping inductor converter is very effective in dealing with fast transient and at the same time it can give better steady state efficiency because a large inductance inductor can be used. Loss during traisient is a concern. This paper attempts to analyze thc losses of the stepping inductor converter in presence of load transients. The Occurrences of load transients in a personal computer mining different kinds of benchmark applications are measured. Then the losses ol‘ the VRMs are compared in terms of the Occurrence of transients measured. I bk-l I q L” :‘.+ f Figure 1. Conceptunl circuit of stepping inductor I nnn S ,  Figure 2. Simplified operating waveform output inductor so that ripple voltage and current is lower aid efficiency is higher, a id  at the same time it can offer a fast transient response of a very low inductance buck converter. The small inductor can be as low as the leakage inductance between the main and auxiliary windings. The simplified circuit diagram is shown in Fig. 1 and the simplified operating waveform is shown in Fig.2. Fig.3 and Fig.4 show the performance improvement by using stepping inductor. The additional parts count in steppmg inductor 11. OPERATION OF STEPPlNti INDUCTOR CONVERTER is low, The conuol of the stepping inductor The stepping inductor converter is a single phase topology for VRM fast transient application. The detailed operation is discussed in [2]. The stepping inductor converter operales in a fashion that in steady state the output inductor is a Iagc inductor and when in transient the large output inductor is “short circuited” and replaced by a very sinall inductor. In this way the stepping inductor converter can beneiit from a large converter is also simple. The main loop-& the same as that in a buck converter. The auxiliary control.loop consists of two comparators only and is separated from the main control loop. It has only one power chainel it does not have the problem of airrent shxiig. During transient the current mismatch in Lo and Lr has to be dissipated or reclaimed. This is the reason that stepping inductor is lossy during transient. 1161 Y __ . . . . . , Figure 4. With slepping inductor 111. DETAILED ANALYSIS OF STEPPING INDUCTOR CONVERTER In the analysis, it is assumed that the main MOSFETs M1 and'M2 are kept m and off correctly in the transient, i.e. M1 off and M2 on in step-down transient and M1 on and M2 off in step-up uansient. It is also assumed that the leakage inductance LI is much smaller than the magnetizing inductance Lm. The implementation of the stepping inductor converter is shown in Fig. 5. Fig. 6 and Fig. 7 show the operating waveforms of the stepping inductor converter in response to a load current step-down ind step-up respectively. The equivalent circuits in different stages in a transient are shown in TABLE I. When there is a transient the output voltage will increase or decrease depending on the type of transient. The switches S1 and S2 are controlled by two hysteresis comparators which monitor the output voltage and turns on S1 or S2 according to the transient. Step-dowii Transient: At t=O in Fig. 6 the load step-down occur and the output voltage begin to rise. At time t l  the output voltage crass the comparator tun  on voltage Vth+W2 and the switch S2 is turned on. This is the beginning of stage I .  After S2 is tumed on, the magnetizing inductor is shorted by a voltage source Vs and the output voltage falls quickly. Note that at the beginning of stage 1 the current in the inductors Lm and L, are equal, but as I t t 3: c t d  . .  ov Rgure 5. hplemenlatlon of stepping inductor t--- - --r---- ---- "0 I I I O t II L2 t3 520N %MI hgure 6 laad step-down trannent lo- - > 0 '  I I I , I e a  Figure 7. L a d  stepup lmnieot different voltages are applied on Lm and 4, the currents in Lm and L, after the beginning of stage 1 will be different. When. the output voltage falls to the comparator tum off voltage Vth-Vh/2 the switch S2 is tumed off and stage 2 begins. In stage 2 because the currents in the leakage inductance iL and magnetizing inductance iM do not match, the mismatch Current will flow in to the reset voltage source Vr, which is implemented by a zener diode and the input voltage. Afta the currents iL and iM match it enters stage 3. If the currents iL and iM have fallen to the level of the current the output voltage would not rise to the nun m threshold of S2 and the transient ends, otherwise S2 will be turned on again and the converter enters stage 1 again and the process repeats until the currents iL and iM equals to the output current. However, it can be shown .that the duration of stage 3 is much shorter than stage 1 and stage 2 so that it can be neglected for analysis. The solutions for output voltage ana inductor currents are shown below: 1162 Load Current Step-up I '  Load Current Step-down I The energy that flows into the shorting voltage source Vs is given by: This is the energy reclaimed in stage 1 for the implementation shown in Fig. 5. The energy that flows into the resetting voltage source Vr is given by: where tid is the time when i.,(t) = ii(t) in stage 3. Depending on the implementation of the resetting voltage source Vr, a portion of this energy is dissipated and the remaining is fed back to the input. This is the loss introduced by stepping inductor during transient. For the step-up transient the same procedure is followed and the corresponding result can be obtained and it is skipped here. To calculate the loss of stepping inductor converter in a load step-down transient an iteration of the above . procedure has to be carried out. The successive energy losses in stage 2 are added and &e sum is the total loss introduced by stepping inductor in response to one load step down transient. IV. EXPERIMENTAL VERIFICATION In order to verify the accuracy of (2) for calculating the loss of the stepping inductor converter, a hardware prototype is built and tested using the following circuit and operation parameters: V;, = 12V, V, = 1.5V, Io-= 35A, Imin = 0.5A, V, = 0.3125V, V, = 2V, V,A = 1.53V, V,, = 50mV, C = 2500pF, L = 3.5pH, L,= 4 h H ,  and R, = lmn. A step load change of 0.5A to 35A at the frequency of 167Hz is applied to the converter. Assuming losses due to step up and step down are equal, the loss contributed from stepping inductor at 167Hz step load change from (2) is 0.41W. The measured total loss from the stepping inductor converter (obtained from subtraction of the average input power without the stepping inductor to the average input power with the .stepping inductor) is 0.49W, which is close to the mathematical prediction. The extra loss may due to the constant housekeeping 1163 power such as fast switching gate drivers, switching and conduction losses of the MOSFETs, and comparators. With the increase of step load frequency or the increase of load step magnitude, the e i~or  of (2) to practical measurement data can bc reduced. . /,'V. REAL WORLD VRM OUTPUT CURRENT TRANSIEN'T MEASUREMENT The multiphase interleaved buck converter improves transient response by inearis of a small equivalent inductance so that output cun-ent slew rate can be high. However ths brings the problem ollarge ripple current in each phase although the effective output ripple current is lower due to interleaving. It cai  be expected that the steady state efficiency of the multiphase interleaved buck converter will be low. On the other hand. the stepping inductor converter allows the use of large inductance while at the same time provides a fast ir,ansient response by additional actions. The efficiency ol the stepping inductor co~iverter is higher hi steady state. However the additional actions in dealing with transient in stepping inductor convater inay cause extra loss. In actual VRM application, efficiency will depend ' on the frequency aid amplitude of the load current transient. In order to compare the efficiency of the VRMs fairly, the effect o l  load transient must be taken into account. A. Methodologv A Pention4 PC is set up and different kinds of benchmark program are rui  on the computer and the output current of the VRM is measured. Afterward the mcurrence of VRM output current transient is recorded. Fig. 8 shows a typical waveform of the VRM output voltage and current Wansient. The specifications of the test PC are listed in TABLE 111. B. Results As shown in Fig. 9, it can be seen that large step change in current is very rare and most of the transient are of low amplitudes. When the transient step size is small the efficiency of the multiphase interleaved buck converter will approach its steady state value. For the stepping inductor converter, low amplitude transient means a shorter recovery time and lower auxiliary switching cycles; both will lead to a smaller loss caused by the extra actions U1 stepping inductor converter. It can be expected that the cnergy loss due to transient hi stepping inductor converter will be low. As a result the overall actual efliciency is expected to be higher than that of the multiphase interleaved converter. To estimale the extra loss of the stepping inductor converter working according to TABLE 111, (2) is used to calculate the sum of loss at different load.step size (both step up and step down conditions and takes the mean of each load step size) and frequency as given in Fig. 9. The estimated extra loss is 4.93W. ~ 1164 . .  ............... , (b) Figure 8. VRM output voltage and current (b) Exaggerated View Chl: VRM Output Voltage Ch2. CN: Gate dnve of high side MOSFETs C h 4  VRM Output Current TABLE 111. TEST PC SPECIFICAT~ONS DDR-SDRAM, CL2.S 3 Phase interleaved buck. L=8ZOnH per phase, fs=19Ok Measured 1 ~ a x i m u i n  Output I I Current Idle State Current I SA 1 I Ooeratine System I Microsoft Windows XP I I I ,  Profession?' I PCMark2002 \ VI. CONCLUSION It has been proveii that the stepping inductor converter has a higher efficiency than a single channel and multiphase inrerleavcd buck converter in steady state 131. hi this paper a way to calculale the transient loss ofa stepping inducror converter is presented so that a complete comparison of the stepping inductor converter to other VRM topologies in real world VRM applications can be obtained. VII. FURTHER WORKS Detailed analysis of losses in the stepping inductor converter and other VRM topologies, including multiphase interleaved buck, under a realistic load profile which reflects the characteristics of modern CPU will be carried out, because the frequency and magnitude of the load transients will directly affect the amount oflosses prluced. Measurements show that the Occurrence of high current transient is very rare. However, further work will illustrate this in a quantitative manner. REFERENCE [ I ]  XiinweiZhoo, Pit-Leong Won& PcngXu, Fred C. Leeand Alex Q.  Iluang. "Investigation of candidate VRM lopologier for future Mmoprocesrors". IEEE Trans. on Power Electrorucs. VaI.13.no.6.Nov.2000.pp.1172-1182. N.K.Poon. C.P.Liu and M.H.Pong"'A low cost DC-DC stepping indwlance vollsge regulvlor with fast transienf loading response". IEEE hoc. APEC'OI. pp.268-272. Y.Y. Law. J.H. Kong, Joe C.P. Liu. N.K. Poan. M.H. Pang. "Comparison of Three Topologios for VRM Fast Transient Application". IEEE Proc. A P E C ' O I ,  pp210-215. [?.I [3] 1165 

Loss analysis of a single phase fast transient VRM converter

http://hub.hku.hk/bitstream/10722/45828/1/106618.pdf

Loss analysis of a single phase fast transient VRM converter

Abstract

Similar works

Full text

Available Versions

HKU Scholars Hub