Causal sampling circuit for measuring reflected voltage and demagnetizing time of transformer
View Patent ↗A causal sampling circuit is developed to measure a reflected voltage and demagnetizing time of the transformer. It includes a signal-generation circuit to generate a sample signal for sampling the reflected voltage of the transformer. A ramp signal of the sampling circuit is generated in response to the demagnetizing of the transformer. A first reference signal is generated in accordance with the magnitude of the ramp signal after the transformer is fully demagnetized. A second reference signal is generated in response to the ramp signal and a bias signal. The sample signal is enabled in response to the demagnetizing of the transformer. The sample signal is disabled once the second reference signal is higher than the first reference signal. A sample-and-hold circuit is coupled to the transformer to sample the reflected voltage of the transformer in response to the sample signal.
1. A sampling circuit of power converter, comprising:
a signal-generation circuit, generating a sample signal for sampling a reflected voltage of a transformer; in which the signal-generation circuit generates a ramp signal in response to a demagnetizing of the transformer; a first reference signal is produced in accordance with a magnitude of the ramp signal once the transformer is fully demagnetized, and a second reference signal is generated in response to the ramp signal and a bias signal; and
a sample-and-hold circuit, coupled to the transformer to generate a voltage signal by sampling the reflected voltage of the transformer in response to the sample signal,
wherein the sample signal is enabled in response to the demagnetizing of the transformer; the sample signal is disabled once the second reference signal is higher than the first reference signal.
2. The sampling circuit as claimed in claim 1 , further comprising a timing circuit to generate a discharge-time signal representing the demagnetizing of the transformer, wherein the discharge-time signal is enabled in response to the demagnetizing of the transformer; the discharge-time signal is disabled once a magnitude of a third reference signal is lower than a magnitude of the voltage signal; in which the third reference signal is correlated to the reflected voltage of the transformer and an offset signal.
3. The sampling circuit as claimed in claim 2 , in which the timing circuit comprises:
an input circuit, coupled to receive the reflect voltage of the transformer to generate the third reference signal in accordance with the reflected voltage of the transformer and the offset signal; and
a latch circuit, for generating the discharge-time signal;
wherein the discharge-time signal is enabled in response to the enable of the time-frame signal, and the discharge-time signal is disabled when the third reference signal is lower than the voltage signal.
4. The sampling circuit as claimed in claim 1 , further comprising a comparison circuit to generate a time-frame signal in response to the reflected voltage of the transformer, wherein the time-frame signal is enabled in response to a disable of a switching signal; the time-frame signal is disabled when the reflected voltage of the transformer is lower than a threshold voltage; in which the switching signal is coupled to drive a power transistor of the power converter for switching the transformer; the enable of the switching signal represents the magnetizing of the transformer; the disable of the switching signal represents a start of demagnetizing the transformer.
5. The sampling circuit as claimed in claim 1 , in which the signal-generation circuit comprises:
a first one-shot circuit, coupled to the switching signal to generate a first pulse signal in response to the disable of the switching signal;
a signal-control circuit, coupled to receive the switching signal and the time-frame signal to generate a reset signal; and
an output circuit, coupled to the first one-shot circuit and the signal-control circuit to generate the sample signal;
wherein the sample signal is enabled in response to the first pulse signal, and the sample signal is disabled in response to the reset signal.
6. The sampling circuit as claimed in claim 5 , in which the signal-generation circuit further comprises:
a second one-shot circuit, coupled to receive the sample signal for generating a hold signal in response to the disable of the sample signal.
7. The sampling circuit as claimed in claim 5 , in which the signal-control circuit comprises:
a first capacitor for generating the ramp signal;
a current source;
a first switch, coupled to the current source and the first capacitor to enable a charge of the first capacitor in response to the enable of the time-frame signal;
a transistor, coupled to the first capacitor to enable a discharge of the first capacitor in response to the switching signal;
a second capacitor, for generating the first reference signal;
a second switch, coupled between the first capacitor and the second capacitor to sample a magnitude of the ramp signal into the second capacitor in response to the disable of the time-frame signal;
a buffer circuit, including generating a bias signal, in which the buffer circuit is coupled to the first capacitor to generate the second reference signal in accordance with the ramp signal and the bias signal; and
a first comparator, coupled to receive the first reference signal and the second reference signal to generate the reset signal when the second reference signal is higher than the first reference signal.
8. The sampling circuit as claimed in claim 1 , in which the sample signal includes a minimum pulse width that is generated in response to the disable of the switching signal.
9. The sampling circuit as claimed in claim 1 , in which the sample-and-hold circuit comprises:
a third capacitor;
a fourth capacitor for generating the voltage signal;
a third switch for sampling the reflected voltage of the transformer into the third capacitor in response to the sample signal; and
a fourth switch coupled to the third capacitor to sample the voltage of the third capacitor into the fourth capacitor in response to the hold signal.
10. A detection circuit of power converter, comprising:
a comparison circuit, coupled to a transformer to generate a time-frame signal in response to a reflected voltage of a transformer, wherein the time-frame signal is disabled once the transformer is fully demagnetized;
a signal-generation circuit, generating a sample signal for sampling the reflected voltage of the transformer, wherein the sample signal is enabled in response to a demagnetizing of the transformer; and the sample signal is disabled before the disable of the time-frame signal is set; and
a sample-and-hold circuit, coupled to the transformer to generate a voltage signal by sampling the reflected voltage of the transformer in response to the sample signal;
wherein the voltage signal is correlated to the output voltage of the power converter.
11. The detection circuit as claimed in claim 10 , further comprising a timing circuit to generate a discharge-time signal representing the demagnetizing of the transformer, wherein the discharge-time signal is enabled in response to the demagnetizing of the transformer; and the discharge-time signal is disabled once the reflected voltage of the transformer is lower than the voltage signal.
12. The detection circuit as claimed in claim 10 , in which the sample-and-hold circuit comprises:
a sample capacitor;
a hold capacitor for generating the voltage signal;
a sample switch for sampling the reflected voltage of the transformer into the sample capacitor in response to the sample signal; and
a hold switch coupled to the sample capacitor to sample the voltage of the sample capacitor into the hold capacitor after the disabling state of the sample signal.