• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Subscribers 63 subscribers
  • Views 903 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Maximum output current of inverting buck boost switcher topologies

Niels Koll
Expert 1950 points
Other Parts Discussed in Thread: TPS54541, TINA-TI

Hi all,

can anybody please tell me how to calculate the maximum achievable output current of a switcher in inverting buck-boost topology?

According to several application notes like e.g. slva317b, the maximum output current depends on the Vin to Vout ratio and the current the switcher and inductor can handle.

and

Switcher efficiency is not considered in these application notes.

In some other application notes, e.g. SNVA807, the efficiency is incorporated in the formula, but I wonder if it is correct.

,

You can see that in case the Vin to Vout ratio is equal to 1/n, your switcher may be able to deliver as much current as you like, your maximum output current is zero.

I am a bit confused about this.

Think of a 100A Switcher with an efficiency of 75% that shall convert +5V to -15V. According to SNVA807, the maximum output voltage is zero, simply nothing, not a single milli-amp.

Can anbody please explain to me of SNVA807 is correct?

And why it is correct?

Regards, Niels

  • 0
    TI__Guru 50120 points

    Hi Neils,

    You can use Equation 2 in the application note SNVA807 which incorporates efficiency into the duty cycle. Of course at the end of the day the  maximum allowable current in an IBB topology will be much less than a buck topology.

    Hopefully this app note provides more guidance and reference regarding your question.

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Expert 1950 points

    Jimmy,

    I'm sorry that the TI web-interface for this forum omitted the pictures I inserted in my last post
    (I added screen snapshots from SNVA807 with CRTL-V, and saw them in the "edit window", but when I pressed the "Post" button, the pics disapeared)

    But back to topic:

    I already had a look into SNVA807.
    My question is if equation (2) of SNVA807 is correct or not.

    According to equation (2) of SNVA807, an IBB switcher with

    • Vin = +5V
    • Vout = -15V

    and 75% efficiency will have a maximum output current of 0 amps as D gets equal 1:

    D=Vout / [ ( Vout-Vin ) * n ]
       = -15V / [ ( -15V - 5V ) * 0.75 ]
       = 1

    and according to equation (1), the max. Output current is:
    Iout = Imax * ( 1 - D )

    As the ( 1 - D ) is zero for D = 1, even a switcher that has an Imax of, let's say, 1000A will not be able to output a single milliamp on his -15V output voltage.

    If this is correct, can you please explain it to me in more detail?

    Regards, Niels

  • 0 in reply to Niels Koll
    TI__Guru 50120 points

    Hi Niels,

    The efficiency factor was included to also take into account the energy dissipation. This is similar to the maximum duty cycle equation used in SLVA477B. The equation you've mentioned presents a formula that does't account for component loses(Section 3 design considerations). In an ideal situation, the efficiency would be 100% which would then allow you to get the maximum current out of the device.The application you've mentioned is also a borderline case which would result in 0A. In most application notes that I've seen, the typical application used are of  high input voltage(12V, 15V, 24V typical) to low negative output voltage conversion. 

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Expert 1950 points

    Jimmy,

    so you say it is correct that a switcher in IBB configuration has the limitation that its maximum Vin-to-Vout ratio must be less that the switcher's efficiency?

    It is hard to believe that a (theoretical) switcher, capable to drive a million amps output current will not generate a single milliamp of Vout=-15V from Vin=+5V when its efficiency is less than 75%.

    I tried to simulate it with TINA-TI and the the TPS54541, but my simulation did not work, even for -1.6V output voltage.

    Regards, Niels

    TPS54541_Vin5V_IBB.tsc

  • 0 in reply to Niels Koll
    TI__Guru 50120 points

    Hi Neil,

    If you are interested in certain data, I've attached a file

    Fullscreen n15V_Efficiency.txt Download 
    Vin	Iin	Pin	Vout	Iout	Pout	Iout	Eff	Iout	Diss
 4.00	-----	8/31/2018	1:24 PM	-----	-----	-----	 4.00	-----	 4.00	-----
3.9975995	0.5042584	2.0158230	-15.0857100	0.1006473	-1.5183364	0.1006473	-75.3209214	0.1006473	3.5341594
3.9978872	0.8346691	3.3369128	-15.0704150	0.1506326	-2.2700955	0.1506326	-68.0298127	0.1506326	5.6070082
3.9978440	0.9206323	3.6805443	-11.9034610	0.2006384	-2.3882909	0.2006384	-64.8896118	0.2006384	6.0688352
3.9978296	0.9036104	3.6124802	-9.1589131	0.2506201	-2.2954081	0.2506201	-63.5410557	0.2506201	5.9078883
3.9978506	0.8842099	3.5349390	-7.2706716	0.3006436	-2.1858812	0.3006436	-61.8364617	0.3006436	5.7208201
3.9978336	0.8632233	3.4510231	-5.8721054	0.3506341	-2.0589605	0.3506341	-59.6623206	0.3506341	5.5099835
3.9979146	0.8414535	3.3640593	-4.7869957	0.4006476	-1.9178982	0.4006476	-57.0114269	0.4006476	5.2819575
3.9978885	0.8213524	3.2836755	-4.0479726	0.4506359	-1.8241618	0.4506359	-55.5524378	0.4506359	5.1078373
4.0459364	0.0758856	0.3070284	-0.5440226	0.1440712	-0.0783780	0.1440712	-25.5279260	0.1440712	0.3854064
4.6285554	0.0794583	0.3677770	-0.5389919	0.1472168	-0.0793486	0.1472168	-21.5752076	0.1472168	0.4471256
4.6078925	0.0798139	0.3677739	-0.5425948	0.1569963	-0.0851854	0.1569963	-23.1624272	0.1569963	0.4529592
4.5918852	0.0785674	0.3607726	-0.5220158	0.1665429	-0.0869380	0.1665429	-24.0977412	0.1665429	0.4477106
4.6413053	0.0744950	0.3457539	-1.8949397	0.0389837	-0.0738717	0.0389837	-21.3654076	0.0389837	0.4196256
4.5921677	0.0725224	0.3330349	-2.0520640	0.0385231	-0.0790520	0.0385231	-23.7368383	0.0385231	0.4120869
4.6206741	0.0712781	0.3293531	-2.2244819	0.0413711	-0.0920292	0.0413711	-27.9424130	0.0413711	0.4213823
4.5623474	0.0735070	0.3353644	-1.9394021	0.0437101	-0.0847715	0.0437101	-25.2774379	0.0437101	0.4201359
4.5698653	0.0739202	0.3378053	-1.8103388	0.0452884	-0.0819873	0.0452884	-24.2705647	0.0452884	0.4197926
4.5666183	0.0776024	0.3543806	-1.7796150	0.0464760	-0.0827093	0.0464760	-23.3391163	0.0464760	0.4370899
4.5724662	0.0728898	0.3332860	-1.6622789	0.0488912	-0.0812708	0.0488912	-24.3847057	0.0488912	0.4145568
 8.00	-----	8/31/2018	1:26 PM	-----	-----	-----	 8.00	-----	 8.00	-----
7.9991441	0.2319392	1.8553148	-15.0824880	0.1006572	-1.5181606	0.1006572	-81.8276547	0.1006572	3.3734753
7.9993926	0.3415291	2.7320250	-15.0814440	0.1506314	-2.2717398	0.1506314	-83.1522338	0.1506314	5.0037647
7.9993350	0.4579541	3.6633281	-15.0772540	0.2006371	-3.0250565	0.2006371	-82.5767290	0.2006371	6.6883846
7.9992762	0.5830321	4.6638352	-15.0703130	0.2506185	-3.7768994	0.2506185	-80.9826940	0.2506185	8.4407346
7.9991951	0.7215673	5.7719579	-15.0587120	0.3006448	-4.5273230	0.3006448	-78.4365223	0.3006448	10.2992809
7.9992409	0.7324563	5.8590945	-12.8803370	0.3506305	-4.5162386	0.3506305	-77.0808294	0.3506305	10.3753331
7.9992357	0.7182503	5.7454533	-10.8952070	0.4006421	-4.3650781	0.4006421	-75.9744768	0.4006421	10.1105313
7.9992383	0.7016336	5.6125344	-9.2983424	0.4506355	-4.1901635	0.4506355	-74.6572436	0.4506355	9.8026978
7.9992775	0.6838179	5.4700491	-7.9877864	0.5006334	-3.9989529	0.5006334	-73.1063444	0.5006334	9.4690020
7.9992187	0.6638695	5.3104374	-6.8783415	0.5506468	-3.7875366	0.5506468	-71.3224978	0.5506468	9.0979740
7.9992357	0.6431067	5.1443623	-5.9334627	0.6006314	-3.5638240	0.6006314	-69.2763027	0.6006314	8.7081863
7.9992749	0.6209482	4.9671355	-5.1113008	0.6506185	-3.3255069	0.6506185	-66.9501950	0.6506185	8.2926424
7.9992854	0.6032441	4.8255218	-4.5310253	0.7019410	-3.1805122	0.7019410	-65.9102244	0.7019410	8.0060341
8.0059623	0.4693360	3.7574866	-3.4771636	0.6209997	-2.1593176	0.6209997	-57.4670741	0.6209997	5.9168041
8.0255513	0.0547690	0.4395511	-2.2571052	0.2671080	-0.6028908	0.2671080	-137.1605542	0.2671080	1.0424419
8.5472337	0.0590987	0.5051305	-1.5025423	0.0459943	-0.0691083	0.0459943	-13.6812798	0.0459943	0.5742388
8.5221721	0.0572295	0.4877195	-1.9198569	0.0472620	-0.0907363	0.0472620	-18.6041936	0.0472620	0.5784558
8.5077040	0.0607233	0.5166155	-1.8560120	0.0487495	-0.0904796	0.0487495	-17.5139122	0.0487495	0.6070950
8.4727690	0.0533834	0.4523054	-1.4364599	0.0587953	-0.0844571	0.0587953	-18.6725957	0.0587953	0.5367625
12.00	-----	8/31/2018	1:27 PM	-----	-----	-----	12.00	-----	12.00	-----
11.9995560	0.1571593	1.8858413	-15.0773180	0.1006571	-1.5176397	0.1006571	-80.4754714	0.1006571	3.4034811
11.9995550	0.2267179	2.7205134	-15.0791900	0.1506368	-2.2714817	0.1506368	-83.4945954	0.1506368	4.9919951
11.9994880	0.2984419	3.5811498	-15.0765280	0.2006386	-3.0249336	0.2006386	-84.4682246	0.2006386	6.6060834
11.9995000	0.3724808	4.4695832	-15.0730770	0.2506164	-3.7775600	0.2506164	-84.5170521	0.2506164	8.2471432
11.9994490	0.4495692	5.3945822	-15.0683400	0.3006393	-4.5301344	0.3006393	-83.9756308	0.3006393	9.9247166
11.9994340	0.5304358	6.3649290	-15.0607870	0.3506291	-5.2807500	0.3506291	-82.9663619	0.3506291	11.6456791
11.9993580	0.6122492	7.3465972	-14.9565490	0.4006375	-5.9921548	0.4006375	-81.5636773	0.4006375	13.3387521
11.9993900	0.6079269	7.2947515	-13.0401520	0.4506342	-5.8763378	0.4506342	-80.5556958	0.4506342	13.1710893
11.9994280	0.5943333	7.1316590	-11.3403830	0.5006334	-5.6773748	0.5006334	-79.6080520	0.5006334	12.8090339
11.9994730	0.5789858	6.9475248	-9.9041692	0.5506459	-5.4536902	0.5506459	-78.4983184	0.5506459	12.4012150
11.9994770	0.5621706	6.7457535	-8.6608514	0.6006298	-5.2019652	0.6006298	-77.1146641	0.6006298	11.9477187
11.9994710	0.5440506	6.5283192	-7.5779706	0.6506165	-4.9303527	0.6506165	-75.5225440	0.6506165	11.4586719
11.9995320	0.5273423	6.3278606	-6.7786427	0.7018907	-4.7578660	0.7018907	-75.1891726	0.7018907	11.0857266
11.9994810	0.5059778	6.0714714	-5.8029918	0.7520674	-4.3642407	0.7520674	-71.8811044	0.7520674	10.4357120
11.9995090	0.4872238	5.8464461	-5.0514441	0.8020742	-4.0516329	0.8020742	-69.3007819	0.8020742	9.8980790
11.9995930	0.4689326	5.6270006	-4.3982012	0.8519750	-3.7471574	0.8519750	-66.5924477	0.8519750	9.3741580
12.0151150	0.4630074	5.5630878	-4.2229911	0.9020430	-3.8093196	0.9020430	-68.4749148	0.9020430	9.3724074
12.0124730	0.0515456	0.6191906	-0.5644995	0.1520845	-0.0858516	0.1520845	-13.8651355	0.1520845	0.7050422
12.4593890	0.0499978	0.6229426	-0.5643298	0.1527258	-0.0861877	0.1527258	-13.8355824	0.1527258	0.7091304
15.00	-----	8/31/2018	1:29 PM	-----	-----	-----	15.00	-----	15.00	-----
15.0001430	0.1285468	1.9282205	-15.0732170	0.1006224	-1.5167037	0.1006224	-78.6582083	0.1006224	3.4449243
15.0001950	0.1830620	2.7459651	-15.0752930	0.1506243	-2.2707055	0.1506243	-82.6924369	0.1506243	5.0166706
15.0001430	0.2392524	3.5888199	-15.0740320	0.2006320	-3.0243326	0.2006320	-84.2709486	0.2006320	6.6131525
15.0000640	0.2966825	4.4502566	-15.0710520	0.2506221	-3.7771395	0.2506221	-84.8746435	0.2506221	8.2273961
15.0000910	0.3557821	5.3367631	-15.0676900	0.3006414	-4.5299711	0.3006414	-84.8823717	0.3006414	9.8667342
15.0000380	0.4166849	6.2502887	-15.0625700	0.3506310	-5.2814035	0.3506310	-84.4985529	0.3506310	11.5316923
15.0000120	0.4800592	7.2008933	-15.0559610	0.4006437	-6.0320756	0.4006437	-83.7684348	0.4006437	13.2329689
14.9999860	0.5423349	8.1350161	-14.9378660	0.4506385	-6.7315781	0.4506385	-82.7481849	0.4506385	14.8665942
15.0000640	0.5388922	8.0834173	-13.2057870	0.5006371	-6.6113065	0.5006371	-81.7885089	0.5006371	14.6947238
15.0000380	0.5266971	7.9004770	-11.6122040	0.5506507	-6.3942679	0.5506507	-80.9352137	0.5506507	14.2947449
15.0000380	0.5128170	7.6922751	-10.2273920	0.6006295	-6.1428734	0.6006295	-79.8576933	0.6006295	13.8351485
14.9999860	0.4972331	7.4584891	-9.0107038	0.6506176	-5.8625228	0.6506176	-78.6020155	0.6506176	13.3210118
15.0000910	0.4837135	7.2557464	-8.0204477	0.7018242	-5.6289440	0.7018242	-77.5791171	0.7018242	12.8846903
15.0000120	0.4639464	6.9592013	-6.9815291	0.7519809	-5.2499766	0.7519809	-75.4393558	0.7519809	12.2091779
15.0000640	0.4462199	6.6933272	-6.1225409	0.8020127	-4.9103556	0.8020127	-73.3619539	0.8020127	11.6036828
15.0000910	0.4299737	6.4496452	-5.3796180	0.8519009	-4.5829011	0.8519009	-71.0566393	0.8519009	11.0325464
14.9999600	0.4142575	6.2138456	-4.7142160	0.9020450	-4.2524351	0.9020450	-68.4348360	0.9020450	10.4662807
15.0000910	0.3956644	5.9350014	-4.0767308	0.9520752	-3.8813541	0.9520752	-65.3976952	0.9520752	9.8163555
15.0088410	0.0459214	0.6892269	-0.5544105	0.1597606	-0.0885730	0.1597606	-12.8510587	0.1597606	0.7777999
18.00	-----	8/31/2018	1:30 PM	-----	-----	-----	18.00	-----	18.00	-----
18.0001610	0.1095303	1.9715627	-15.0662000	0.1006175	-1.5159238	0.1006175	-76.8894568	0.1006175	3.4874865
18.0001350	0.1549117	2.7884321	-15.0702750	0.1506264	-2.2699819	0.1506264	-81.4071073	0.1506264	5.0584139
18.0001870	0.2008518	3.6153700	-15.0698800	0.2006345	-3.0235374	0.2006345	-83.6300965	0.2006345	6.6389073
18.0002920	0.2479426	4.4630385	-15.0677660	0.2506199	-3.7762819	0.2506199	-84.6123527	0.2506199	8.2393203
18.0002650	0.2960324	5.3286622	-15.0651940	0.3006483	-4.5293247	0.3006483	-84.9992832	0.3006483	9.8579869
18.0002390	0.3452595	6.2147533	-15.0611190	0.3506354	-5.2809609	0.3506354	-84.9745854	0.3506354	11.4957142
18.0001870	0.3959249	7.1267226	-15.0560370	0.4006478	-6.0321684	0.4006478	-84.6415490	0.4006478	13.1588910
18.0001080	0.4483094	8.0696183	-15.0495420	0.4506409	-6.7819395	0.4506409	-84.0428775	0.4506409	14.8515578
18.0001080	0.4803533	8.6464118	-14.3768770	0.5006403	-7.1976444	0.5006403	-83.2442937	0.5006403	15.8440563
18.0001610	0.4758470	8.5653232	-12.8180990	0.5506536	-7.0583318	0.5506536	-82.4059025	0.5506536	15.6236550
18.0001350	0.4687861	8.4382134	-11.4362920	0.6006313	-6.8689946	0.6006313	-81.4034230	0.6006313	15.3072080
18.0002390	0.4574162	8.2336002	-10.1642250	0.6506161	-6.6130087	0.6506161	-80.3173408	0.6506161	14.8466089
18.0001870	0.4433467	7.9803231	-8.9880965	0.7018676	-6.3084534	0.7018676	-79.0501003	0.7018676	14.2887766
18.0002130	0.4282894	7.7092995	-7.9504906	0.7519305	-5.9782162	0.7519305	-77.5455175	0.7519305	13.6875157
18.0002130	0.4127758	7.4300530	-7.0172284	0.8019374	-5.6273781	0.8019374	-75.7380611	0.8019374	13.0574312
18.0001870	0.3976422	7.1576347	-6.1981279	0.8519119	-5.2802589	0.8519119	-73.7710024	0.8519119	12.4378935
18.0002130	0.3835269	6.9035655	-5.4718118	0.9020687	-4.9359503	0.9020687	-71.4985655	0.9020687	11.8395159
18.0002920	0.3696556	6.6539080	-4.8116169	0.9519614	-4.5804734	0.9519614	-68.8388448	0.9519614	11.2343814
18.0002390	0.3517542	6.3316593	-4.1566101	1.0019673	-4.1647874	1.0019673	-65.7771872	1.0019673	10.4964467
21.00	-----	8/31/2018	1:32 PM	-----	-----	-----	21.00	-----	21.00	-----
21.0011450	0.0966125	2.0289735	-15.0603040	0.1006195	-1.5153609	0.1006195	-74.6860860	0.1006195	3.5443343
21.0009880	0.1350770	2.8367498	-15.0649010	0.1506288	-2.2692083	0.1506288	-79.9932458	0.1506288	5.1059581
21.0011180	0.1739717	3.6535992	-15.0654230	0.2006336	-3.0226299	0.2006336	-82.7302005	0.2006336	6.6762291
21.0010140	0.2137837	4.4896738	-15.0642640	0.2506233	-3.7754553	0.2506233	-84.0919716	0.2506233	8.2651291
21.0010400	0.2544898	5.3445505	-15.0617040	0.3006478	-4.5282679	0.3006478	-84.7268240	0.3006478	9.8728183
21.0009620	0.2959222	6.2146502	-15.0586610	0.3506328	-5.2800612	0.3506328	-84.9615186	0.3506328	11.4947115
21.0009350	0.3383365	7.1053837	-15.0543180	0.4006473	-6.0314722	0.4006473	-84.8859460	0.4006473	13.1368558
21.0008830	0.3818847	8.0199163	-15.0489820	0.4506370	-6.7816286	0.4506370	-84.5598418	0.4506370	14.8015449
21.0009350	0.4245021	8.9149408	-14.9616940	0.5006369	-7.4903767	0.5006369	-84.0204873	0.5006369	16.4053175
21.0009620	0.4253869	8.9335352	-13.5233810	0.5506494	-7.4466419	0.5506494	-83.3560484	0.5506494	16.3801771
21.0008570	0.4206198	8.8333771	-12.1385040	0.6006360	-7.2908230	0.6006360	-82.5372095	0.6006360	16.1242001
21.0009350	0.4139708	8.6937732	-10.8977540	0.6506165	-7.0902586	0.6506165	-81.5555959	0.6506165	15.7840318
21.0009880	0.4056226	8.5184756	-9.7634314	0.7017213	-6.8512077	0.7017213	-80.4276261	0.7017213	15.3696832
21.0010400	0.3959564	8.3154956	-8.7327371	0.7519638	-6.5667026	0.7519638	-78.9694680	0.7519638	14.8821982
21.0010400	0.3837786	8.0597497	-7.7786794	0.8019244	-6.2379127	0.8019244	-77.3958614	0.8019244	14.2976625
21.0009350	0.3707565	7.7862338	-6.9022143	0.8518998	-5.8799953	0.8518998	-75.5178359	0.8518998	13.6662290
21.0010660	0.3576073	7.5101339	-6.1177064	0.9020637	-5.5185609	0.9020637	-73.4815254	0.9020637	13.0286948
21.0010400	0.3453595	7.2529091	-5.4183832	0.9519274	-5.1579073	0.9519274	-71.1150133	0.9519274	12.4108164
21.0009350	0.3320010	6.9723310	-4.7576315	1.0020021	-4.7671568	1.0020021	-68.3724963	1.0020021	11.7394878
    of the automated efficiency test data I ran for LMZM23601-IBB application. This test sweeps Iout for each Vin setting at Vout = -15V. The case I ran here Vin=4V, Vout= -15V which is very close to your example application. The data does indeed show that you can pull up to 150mA before output voltage regulation tanks. The equation from the application note still applies for most of the dataset presented other then the first two voltages(4V and 8V). In the case of the first two voltages, it would appear that without the efficiency factor,  D = -Vout / (-Vout - Vin) is more appropriate in these instances.

    I will have to look further into this to see how to accommodate for this. 

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Expert 1950 points

    Hi Jimmy,

    thanks for your reply. Your efficiency test data is very interesting.
    My assumption is that the efficiency factor (n) should better be used to weight (1-D), and not D.
    But this is only my assumption.

    I am looking forward to your answer.

    Regards, Niels

  • 0 in reply to Niels Koll
    TI__Guru 50120 points
    Hi Niels,

    The equation I have is just an estimation of what the limitation the IBB topology will have. In testing the border line cases like this, you will be able to push some amount, definitely more than a few milliamps. However I wanted to encompass a wide range of operation and applications while still be conservative enough for me to recommend a design that is proper and well regulated. In actuality the equation should take into account inductor loses and switching loses which would greatly complicate the equation. If the device is in a hot environment, the DCR losses will be greater which will result in worsen conditions. To air on the safe side I decided to weight D rather than (1-D) but I do see that there is a theoretical limitation to this equation.

    If you have a specific application that you care about, please let me know and I can test it to see if there are any limitations.

    Regards,
    Jimmy