00:00:00.110 [Music] 00:00:07.15000:00:07.160 all right Valerie I'm ready I have my 00:00:10.00900:00:10.019 charge hat I'm here and I'm ready to 00:00:12.04900:00:12.059 calculate the capacitance for a boost 00:00:15.20000:00:15.210 converter what's that oh we don't use 00:00:22.42900:00:22.439 the charge hat approach for the boost 00:00:25.04000:00:25.050 converter okay right we don't use it for 00:00:31.60900:00:31.619 the boost converter we actually do a 00:00:33.11000:00:33.120 different calculation and we're going to 00:00:34.45900:00:34.469 talk about how we do that the boost 00:00:36.77000:00:36.780 converter has the inductor at the input 00:00:39.04900:00:39.059 here so the calculation for the 00:00:43.33000:00:43.340 capacitance is actually going to be 00:00:45.13900:00:45.149 independent of the inductance so let's 00:00:48.61900:00:48.629 look at when the switch is on where the 00:00:50.51000:00:50.520 current is flowing so when this switch 00:00:52.58000:00:52.590 is on current flows through here in this 00:00:56.38900:00:56.399 loop and because we need to provide 00:00:58.84900:00:58.859 power to the load they're also gonna 00:01:01.01000:01:01.020 have current flowing in this direction 00:01:03.59000:01:03.600 so capacitor charge is going to be 00:01:07.85000:01:07.860 supplied to the load during the switch 00:01:11.00000:01:11.010 on duty time and when it's off its going 00:01:14.57000:01:14.580 to go to the other phase but from this 00:01:17.06000:01:17.070 we know that the capacitor has to uphold 00:01:19.16000:01:19.170 the voltage within a certain limit and 00:01:23.24000:01:23.250 we're gonna assume that you know you're 00:01:25.78900:01:25.799 given a Delta Delta V out that is your 00:01:32.39000:01:32.400 target ripple alpha voltage ripple so 00:01:35.99000:01:36.000 assuming you have a voltage ripple 00:01:37.60900:01:37.619 that's your target you want to pick a 00:01:39.49900:01:39.509 capacitor to ensure that your voltage 00:01:41.60000:01:41.610 stays within that ripple this value or 00:01:44.14900:01:44.159 below so to do that we need to 00:01:46.91000:01:46.920 understand how the current is flowing 00:01:49.03000:01:49.040 this is the current for the capacitor 00:01:52.78900:01:52.799 during the two stages so during the 00:01:56.03000:01:56.040 first stage and this current is going 00:01:58.24900:01:58.259 into the capacitor so during the first 00:02:00.38000:02:00.390 stage the current is coming out of the 00:02:02.38900:02:02.399 capacitor so we actually have a negative 00:02:04.37000:02:04.380 value and we know that the output 00:02:06.31900:02:06.329 current has to be maintained at whatever 00:02:08.02900:02:08.039 the average value is so actually that 00:02:10.13000:02:10.140 current is going 00:02:11.64000:02:11.650 the average output current here this 00:02:14.39900:02:14.409 level and then during the other phase 00:02:16.25900:02:16.269 it's going to put all that charge that 00:02:18.30000:02:18.310 it that came out of the capacitor has to 00:02:21.17900:02:21.189 go back into the capacitor to charge it 00:02:23.13000:02:23.140 up to the same level so this charge in 00:02:26.13000:02:26.140 this charge is actually going to balance 00:02:27.53900:02:27.549 but for this calculation of the hotter 00:02:30.21000:02:30.220 side the capacitor we actually can look 00:02:32.36900:02:32.379 just at this value we can start with the 00:02:34.94900:02:34.959 capacitor equation so IC equals c dv/dt 00:02:41.17900:02:41.189 and we're going to be looking at just 00:02:43.71000:02:43.720 the on time of the on switch phase so 00:02:48.96000:02:48.970 from 0 to DT here and that means we can 00:02:52.71000:02:52.720 make these Delta's instead of DS so we 00:02:54.93000:02:54.940 can see Delta V so the change in voltage 00:02:57.47900:02:57.489 over delta T the change in a time and we 00:03:01.92000:03:01.930 want to calculate the capacitance value 00:03:03.56900:03:03.579 so let's move this equation around our 00:03:06.44900:03:06.459 capacitance value it's going to be equal 00:03:08.81900:03:08.829 to the current IC times delta T divided 00:03:13.61900:03:13.629 by Delta V if we apply then the 00:03:17.72000:03:17.730 conditions for the switch on phase we 00:03:22.47000:03:22.480 will see that I see is gonna become this 00:03:25.94900:03:25.959 IO and here we'll just I oh it's 00:03:32.75900:03:32.769 actually gonna be negative but the Delta 00:03:34.28900:03:34.299 V is also gonna be negative so those 00:03:35.90900:03:35.919 will cancel out and then your D this 00:03:39.14900:03:39.159 will become DT and then your Delta V is 00:03:42.05900:03:42.069 gonna be your Delta V out which is your 00:03:44.67000:03:44.680 value that was given to you or the 00:03:46.71000:03:46.720 ripple value that you want so here you 00:03:50.64000:03:50.650 just need to know your duty ratio your 00:03:53.55000:03:53.560 period and the output current in order 00:03:56.75900:03:56.769 to figure out the capacitance that you 00:03:59.21900:03:59.229 need to achieve that ripple if you want 00:04:02.12900:04:02.139 to be more specific to this resistor we 00:04:05.12900:04:05.139 can simply put that value in we can also 00:04:09.83900:04:09.849 write it as this value instead of IO we 00:04:13.28900:04:13.299 can write it as V out ever to be out 00:04:16.00900:04:16.019 over R and then DT over Delta V out so 00:04:24.86000:04:24.870 this is the equation that you can use to 00:04:27.99000:04:28.000 determine the capacitor value that you 00:04:30.57000:04:30.580 want for a given change in the output to 00:04:35.01000:04:35.020 the voltage ripple and it depends on 00:04:37.40900:04:37.419 your output current which is your output 00:04:41.36900:04:41.379 voltage divided by your resistor and DT 00:04:44.33900:04:44.349 so there's no charge hat here it's just 00:04:48.05900:04:48.069 a straight calculation and this will 00:04:50.70000:04:50.710 help you give you the value for the 00:04:52.70900:04:52.719 Kresser that you want some students get 00:04:55.08000:04:55.090 confused because in reality this is 00:04:58.32000:04:58.330 actually an RC circuit so as the current 00:05:01.89000:05:01.900 is flowing through it's going to be 00:05:04.20000:05:04.210 decreasing in voltage so if you wanted 00:05:06.45000:05:06.460 to actually write it out or draw it out 00:05:09.74900:05:09.759 more accurately it might be decreasing 00:05:11.79000:05:11.800 like this say so there is some change in 00:05:16.80000:05:16.810 the current value over time but because 00:05:20.55000:05:20.560 we're dealing with averages the average 00:05:23.12900:05:23.139 value that's going to come out of that 00:05:24.14900:05:24.159 is still going to be the average output 00:05:25.95000:05:25.960 current so we approximate that as the 00:05:29.01000:05:29.020 average value and we do not do all the 00:05:31.80000:05:31.810 integration of the slight change in the 00:05:35.12900:05:35.139 output current so it is there in the 00:05:37.26000:05:37.270 waveform there will be some difference 00:05:38.85000:05:38.860 in reality but when we're doing the 00:05:40.70900:05:40.719 calculation we will simply use the 00:05:44.18900:05:44.199 average value and calculate it like this
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