Boost converter design issue

Thread Starter

lbranco93

Joined Sep 7, 2022
3
For a circuit I'm designing, I want to use an MC34063AL-D08-T DC/DC converter (or a similar one; I just need a through-hole package) as a boost converter to boost a voltage \( V_{in} = 3.3-4.2 V \) (coming from a Li-ion battery) to a stable output of \( V_{out} = 5 V \), with \( I_{out} = 250 mA \).
I'm quite new to electronics and circuit design and I have no experience working with buck-boost converters, so I have been studying the datasheet, and the examples provided in it, in order to understand which components I'll need for my use case.

The issue is that the provided examples don't make sense to me. Here is the boost converter schematic provided in the datasheet:

Capture.PNG

If I try to use the calculation table provided at the end of the datasheet, the component values don't match the ones shown in the schematic above.

Capture2.PNG

For example, the datasheet specifies that this configuration has a voltage ripple \( V_{\text{ripple}} = 400 mV \) (highlighted). If I try to compute the \( V_{\text{ripple}} \) using the same components as in the schematic, it works out to around \( 194 mV \).

I can't even seem to reproduce the schematic; using the same requirements
\[
V_{in} = 12 V, V_{out} = 28 V, I_{out} = 175 mA
\]
all the components turn out different from what is shown.
What am I missing?

Note: the diode used in the schematic is a 1N5819; I assumed a forward voltage of about \( V_{F} = 0.45 V \) at a forward current of about \( I_{F} = 250 mA \). I won't use this diode, but the \( V_{F} \) is needed to compute the duty cycle of the converter.
 

Ian0

Joined Aug 7, 2020
6,294
Because energy is only supplied to the 5V side whilst the transistor is off, the ripple can be estimated from V=It/C, where t is the on time.
What frequency are you running it at?
 

Thread Starter

lbranco93

Joined Sep 7, 2022
3
That's what I did to estimate the ripple, the only difference is that the formula provided in the table has a factor of 9.
The point is that I'm using their formulas, provided in the table (second picture), but the values I get don't match the components they used in their own example (first picture).

I'm not really sure about the frequency, I did my calculations with 40kHz as a test but it's not critical. In the example provided in the datasheet, it seems they are using about 22kHz
 

Thread Starter

lbranco93

Joined Sep 7, 2022
3
Because energy is only supplied to the 5V side whilst the transistor is off, the ripple can be estimated from V=It/C, where t is the on time.
What frequency are you running it at?
That's what I did to estimate the ripple, the only difference is that the formula provided in the table has a factor of 9.
The point is that I'm using their formulas, provided in the table (second picture), but the values I get don't match the components they used in their own example (first picture).

I'm not really sure about the frequency, I did my calculations with 40kHz as a test but it's not critical. In the example provided in the datasheet, it seems they are using about 22kHz
 

eetech00

Joined Jun 8, 2013
3,418
For a circuit I'm designing, I want to use an MC34063AL-D08-T DC/DC converter (or a similar one; I just need a through-hole package) as a boost converter to boost a voltage \( V_{in} = 3.3-4.2 V \) (coming from a Li-ion battery) to a stable output of \( V_{out} = 5 V \), with \( I_{out} = 250 mA \).
I'm quite new to electronics and circuit design and I have no experience working with buck-boost converters, so I have been studying the datasheet, and the examples provided in it, in order to understand which components I'll need for my use case.

The issue is that the provided examples don't make sense to me. Here is the boost converter schematic provided in the datasheet:

View attachment 275793

If I try to use the calculation table provided at the end of the datasheet, the component values don't match the ones shown in the schematic above.

View attachment 275794

For example, the datasheet specifies that this configuration has a voltage ripple \( V_{\text{ripple}} = 400 mV \) (highlighted). If I try to compute the \( V_{\text{ripple}} \) using the same components as in the schematic, it works out to around \( 194 mV \).

I can't even seem to reproduce the schematic; using the same requirements
\[
V_{in} = 12 V, V_{out} = 28 V, I_{out} = 175 mA
\]
all the components turn out different from what is shown.
What am I missing?

Note: the diode used in the schematic is a 1N5819; I assumed a forward voltage of about \( V_{F} = 0.45 V \) at a forward current of about \( I_{F} = 250 mA \). I won't use this diode, but the \( V_{F} \) is needed to compute the duty cycle of the converter.
I believe the values shown in the table are actual HW tested results. You can't really expect them to match the calculations due to component tolerances, etc.

BTW - 1N5819 Vf @ 175mA is 0.3v according to OnSemi datasheet.
 
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