NPN high-voltage transistor in a SOT323 plastic package. PNP complement: PMST5401. PINNING PIN 1 2 3 PMST5550; PMST5551 DESCRIPTION base emitter collector handbook, halfpage 3 3 1 MARKING TYPE NUMBER PMST5550 PMST5551 Note 1. ∗ = - : Made in Hong Kong. ∗ = t : Made in Malaysia. Fig.1 Simplifie.