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I am designing an SMPS (single switch high-PFC flyback in voltage mode inspired by an article) around the voltage-mode IC KA7553. However the datasheet for this part is really short and skimps on some details, including how the oscillator frequency is set. Here is a diagram of the IC:

Internal diagram of IC KA7553

I managed to understand most of the functions except the oscillator section for which no formula or explanation is given. The datasheet only mentions a frequency of 135kHz for C_T=360pF with no R_T and a note mentions that R_T should be between 3.3k and 10k corresponding to frequencies of 5kHz to 600kHz with unspecified capacitance for C_T.

I found no application documentation for this IC besides the datasheet. Thus, considering the topology in the IC, I assumed that R_T would be setting a current in a current mirror which is then applied to C_T to generate the ramp, like for the TL494. This would yield a frequency given by f=1/(C_T R_T). However, I also found an application note that applies this IC and which claims an operating frequency of 100kHz with C_T=390pF and R_T=6.2k. This does not match with the assumed formula for the frequency.

Thus, I do not understand how the designer is supposed to select an operating frequency. My questions are as follows:

  • Is there a standard oscillator scheme from the time the IC was introduced the user is supposed to know to use it?
  • Is the user expected to characterize this on the bench?
  • Do anyone know how the frequency is set with this IC?
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I am attempting to repair an SMPS which uses the KA7552. I'm an inexperienced hobbyist, so I struggled with the lack of information in the KA7552 datasheet. I spotted a brief article on seekic.com, which presents a typical application circuit for KA7552, but also helpfully says that "the FA5311 power IC's electrical specifications is nearly the same with this IC". I looked up the datasheet of said FA5311. It is much more informative. I don't know how precisely similar to KA7552 this alternative part is, but it provides a formula to calculate the \$C_t / R_t\$ oscillation frequency:

$$f = \frac{10^6}{4 R_t C_t}$$

[\$f\$ in kHz, \$R_t\$ in kohms, \$C_t\$ in pF]

If you plug in \$C_T=390\text{ pF}\$ and \$R_T=6.2\text{ k}\$ the derived frequency is 103 kHz, which resembles the 100 kHz you found in your application note.

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