JAJSNN2 データシート

JAJSNN2B October 2023 – July 2024 UCC25660

PRODUCTION DATA

パッケージ・オプション

メカニカル・データ（パッケージ｜ピン）

DDB|14
- MPDC008

サーマルパッド・メカニカル・データ

発注情報

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8.2.2.19 OVP/OTP Pin

The OVP/OTP is used for protecting the power stage from over voltage. Also, the same pin is also used for over temperature protection using negative temperature coefficient (NTC) thermistor. As the bias winding voltage is the mirror image of the output voltage through the turns ratio of the transformer, pulling up this pin with a zener diode is a convenient approach to set the OVP on the primary side. In this design, the nominal output voltage is 12V. Both the bias winding and the secondary side winding has 2 turns. Assuming there is a 0.5V drop in the rectifier diodes (Vf) and a further 0.5V drop due to other losses (Vloss), the nominal voltage of the bias winding is given by:

Equation 63.

V_{B i a s W i n d i n g N o m} = (12 + 0.5 + 0.5) \cdot \frac{N_{a u x}}{N_{2}} = (12 + 0.5 + 0.5) \cdot \frac{2}{2} = 13 V

The desired OVP threshold in this design is 140% of the nominal value. The OVP threshold level (V_OVPpos) in UC25660 device is 3.5V.

The required voltage rating of the Zener diode is then given by:

Equation 64.

V_{z} = (1.4 \cdot V_{o u t} + V_{d r o p}) \cdot \frac{N_{a u x}}{N_{2}} - V_{O V P p o s} = (1.4 \cdot 12 + 0.5 + 0.5) \cdot \frac{2}{2} - 3.5 = 14.3 V

Assume actual voltage rating of Zener used is 15V.

Then actual output voltage at which OVP will be triggered is

Equation 65.

V_{o u t_o v p} = (V_{z} + V_{O V P p o s}) \cdot \frac{N_{2}}{N_{a u x}} - V_{d r o p} = (15 + 3.5) \cdot \frac{2}{2} - 1 = 17.5 V = 146 % \cdot V_{o u t}

During normal operation, the voltage of the OVP/OTP pin should be within the working window of 0.8V to 3.5V. For over temperature protection, the OVP/OTP pin should be pulled down below OTP threshold of 0.8V.

At room temperature, the OVP/OTP pin voltage is considered as 1.4V. So, at room temperature, the effective resistance value at this pin should be

Equation 66.

R_{O V P / O T P_25} = \frac{1.4 V}{I_{O V P_O T P}} = \frac{1.4 V}{100 \cdot 10^{- 6} A} = 14 k Ω

Equation 67.

R_{O V P / O T P_25} = \frac{R_{e x t} \cdot R_{N T C_25}}{R_{e x t} + R_{N T C_25}} = 14 k Ω

where R_ext is external resistor that is in parallel with the thermistor. And R_{NTC_25} is resistance value of the thermistor at the room temperature.

For this design, over temperature protection is set at the 110⁰C. So based on the availability and temperature coefficient of NTCs,

Equation 68.

\frac{R_{N T C_110}}{R_{N T C_25}} = 0.035263

(refer B57371V2474J060 Datasheet) is chosen. Here R_{NTC_110} is the resistance of the thermistor at the 110⁰C.

For OTP trigger, the OVP/OTP pin voltage should be below 0.8V.

Equation 69.

R_{O V P / O T P_110} = \frac{0.8 V}{I_{O V P_O T P}} = \frac{0.8 V}{100 \cdot 10^{- 6} A} = 8 k Ω

Equation 70.

R_{O V P / O T P_110} = \frac{R_{e x t} \cdot R_{N T C_110}}{R_{e x t} + R_{N T C_110}} = 8 k Ω

From equations,Equation 67, Equation 68, Equation 70, R_{NTC_25} and R_ext are obtained as 510kΩ and 14.4kΩ. So, finally R_{NTC_25}=470kΩ (Manfacturer part number: B57371V2474J060 ) and R_ext=15kΩ are chosen.

So, at room temperature, with new chosen resistors, the OVP/OTP voltage will be

Equation 71.

R_{O V P / O T P_25} \cdot I_{O V P_O T P} = (\frac{15 k \cdot 470 k}{15 k + 470 k}) \cdot 100 \cdot 10^{- 6} = 1.454 V

At 110⁰C, the OVP/OTP voltage will be

Equation 72.

R_{O V P / O T P_110} \cdot I_{O V P_O T P} = (\frac{15 k \cdot (470 k \cdot 0.035263)}{15 k + (470 k \cdot 0.035263)}) \cdot 100 \cdot 10^{- 6} = 0.78 V