Flyback transformer keeps saturating and I can't see why

My 65 W flyback works fine at room temp but saturates after ten minutes under load, and the core doesn't feel that hot. I gapped it per the datasheet and the primary inductance measures right. Could this be core loss heating I can't feel, or is my turns count just marginal? What did you check first?

“带载十分钟后才饱和”这种时序，最先怀疑的不应该是磁芯本身，而是热机后峰值电流在漂移。MOSFET 的 Rdson 随温度上升、电流采样电阻温漂，都会让峰值电流限制点慢慢上移，B 峰就一点点往 Bsat 上贴，冷机时看着一切正常。

排查顺序建议：

1. 上电流探头看初级电流波形，对比冷机和热机十分钟后的峰值电流有没有涨，波形顶部有没有变尖。变尖就是进饱和的前兆，比摸磁芯温度准得多。

2. 算实际 Bmax = Lp × Ip / (Np × Ae)。注意两点：一是 Ip 要用热机后的实测值，不是设计值；二是铁氧体的 Bsat 随温度下降很明显，100°C 时能比常温掉 20% 以上。余量至少留 30%，卡着 Bsat 设计必翻车。

3. 如果是 CCM 工作，还要查斜率补偿。补偿不够会出现次谐波振荡，占空比逐周期摆动，个别周期的伏秒失衡会把磁芯直接推到饱和，这种在冷机时可能刚好不触发。

磁芯表面不烫不代表内部不烫，损耗热点在磁芯中柱里面，手根本摸不到。先按上面三步把电参数抓出来，基本就能定位。

Have you tried looking at it from the signal integrity angle? That's how we found our root cause.