Flyback converter output current evaluation circuit and evaluation method

Patent metadata

Field	Value
Publication number	US-9954449-B2
Application number	US-201615343091-A
Country	US
Kind code	B2
Filing date	Nov 3, 2016
Priority date	Dec 17, 2014
Publication date	Apr 24, 2018
Grant date	Apr 24, 2018

Abstract

An output current calculating circuit for a flyback converter operating under CCM and DCM is disclosed. The off current value I OFF and the blanking current value I LEB flowing through a sensing resistor are calculated using a detection module and are summed together using a current summing unit. A voltage converted from the sum value of the off current value I OFF and the blanking current value I LEB is transmitted through an output stage in a predetermined time ratio of a cycle with the duty cycle determined by a logic control unit, in which the logic control unit controls the output stage to receive the voltage converted from sum current in a predetermined time period of each cycle, and prevents the output stage to receive the voltage converted from sum current in the remaining time other than such predetermined time period of each cycle.

First claim

The invention claimed is:

1. An output current calculation circuit for a flyback converter comprises:
a detection module detecting a primary current flowing through a sensing resistor connected in series with a primary winding, wherein the detection module comprising a first voltage-current converter collecting a sensing voltage signal across the sensing resistor representing the primary current, and converting the sensing voltage signal into an intermediate current flowing through a conversion resistor connected between an output terminal of the first voltage-current converter and ground;
a sample and hold latch storing a first current value and a second current value;
a current summing unit summing the first current value and the second current value;
an output stage transmitting a voltage converted from a sum current of the first current value and the second current value according to a predetermined ratio in each cycle; and
a logic control unit controlling the output stage only to transmit the voltage converted from the sum current within a predetermined time period in each switching cycle.

2. The output current calculation circuit for the flyback converter of claim 1 , wherein the detection module further comprises:
a first voltage follower having a positive input terminal connected to the output terminal of the first voltage-current converter;
a first switch driven by a first signal connected between an output terminal of the first voltage follower and a first storage capacitor of the sample and hold latch;
a second voltage follower having a positive input terminal connected to the output terminal of the first voltage-current converter; and
a second switch driven by a second signal connected between an output terminal of the second voltage follower and a second storage capacitor of the sample and hold latch.

3. The output current calculation circuit for the flyback converter of claim 2 , wherein the first signal is a control signal driving a main switch controlling on or off of the primary winding, the second signal is a leading-edge blanking signal used for shielding an initial spike of the primary current.

4. The output current calculation circuit for the flyback converter of claim 3 , wherein the first current value corresponds to an off current value I OFF flowing through the sensing resistor at a moment the main switch of the primary winding is turned off, and the second current value corresponds to a blanking current value I LEB flowing through the sensing resistor at a moment an active state of a leading-edge blanking signal used for shielding the initial spike of the primary current ends.

5. The output current calculation circuit for the flyback converter of claim 4 , wherein the current summing unit comprises:
a second voltage-current converter that restores the sensing voltage signal corresponding to the off current value I OFF stored in the sample and hold latch to a current equal to the off current value I OFF flowing through an output terminal of the second voltage-current converter;
a third voltage-current converter that restores and converts the sensing voltage signal corresponding to the blanking current value I LEB and stored in the sample and hold latch into a current equal to the blanking current value I LEB flowing through an output terminal of the third voltage-current converter; and
a third voltage follower; wherein output currents from the second and the third voltage-current converters are connected to a common node and flow through a summing resistor between the common node and the ground, and the third voltage follower has a positive input terminal connected to the common node, and an output terminal delivering voltage V TRS equals to (I LEB +I OFF ) multiplying by resistance value R SUM of the summing resistor.

6. The output current calculation circuit for the flyback converter of claim 5 , wherein the output stage comprises a third switch having an input terminal receiving the voltage V TRS generated by the sum current and an output terminal grounded;
wherein the third switch is turned off during a predetermined main switch off period T OFF in each switching cycle, such that the output stage outputs the voltage V TRS , and the third switch is turned on during a remaining period of each switching cycle in order to release the voltage V TRS to the ground to prevent the output stage from transmitting the voltage V TRS .

7. The output current calculation circuit for the flyback converter of claim 6 , wherein when the flyback converter operates under a Continuous Current Mode (CCM), during the main switch off period T OFF the third switch is turned off and the voltage V TRS is transmitted to the output terminal of the output stage, while during the main switch on period T ON the third switch is turned on to prevent the output stage from transmitting voltage V TRS , and an output voltage V F from output stage satisfies the following equation:

V
F

=

V
TRS

×

T
OFF

T
ON

+

T
OFF

.

8. The output current calculation circuit for the flyback converter of claim 7 , wherein a ratio between a number of windings N P for primary windings and a number of windings N S for secondary windings is n, and an output current I O delivered from the flyback converter to a load satisfies the following equation:

I
O

=

n
×

1
2

×

V
F

R
SUM

.

9. The output current calculation circuit for the flyback converter of claim 6 , wherein when the flyback converter operates under a Discontinuous Current Mode (DCM), during the main switch off period T OFF the third switch is turned off and the voltage V TRS is transmitted to the output terminal of the output stage, during the main switch on period T ON and a dwell time T D the third switch is turned on to prevent the output stage from transmitting voltage V TRS , and an output voltage V F from output stage satisfies the following equation:

V
F

=

V
TRS

×

T
OFF

T
ON

Assignees

• Alpha & Omega Semiconductor Cayman Ltd

Inventors

• Chen Yu-Ming
• Cheng Jung-Pei
• Huang Pei-Lun

Classifications

• G01R19/15

  Indicating the presence of current · CPC title

• G01R19/25

  using digital measurement techniques · CPC title

• H02M3/33515 Primary

  with digital control · CPC title

• G01R19/10

  Measuring sum, difference or ratio · CPC title

• H02M1/08

  Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters · CPC title